Molecular simulation study of the quantum effects of hydrogen adsorption in metal-organic frameworks: influences of pore size and temperature

In this work a systematic molecular simulation study was performed to investigate the influence of pore size and temperature on the quantum effects of hydrogen adsorption in metal-organic frameworks (MOFs) with temperature varied from 40 to 120 K. To do this, three isoreticular MOFs (IRMOFs) with different pore sizes were adopted, and quadratic Feynman–Hibbs (FH) effective potential was introduced to consider the quantum effect. The results show that quantum effects diminish with increasing pore size of IRMOFs at lower pressure (loading), while the opposite trend appears at higher pressure (loading). Through the simulations it is also found that the quantum effects may be dominantly determined by the adsorbate–adsorbate or adsorbate–MOFs interactions with the varying of pressure (loading). In addition, the results also indicate how the temperature influences the quantum effects of H₂ adsorption in MOFs within the pressure range considered.     

Computer simulation of gas adsorption in modified COF-108: the impregnation of C60 into COF-108

Gas adsorption and separation performance of COF-108 framework impregnated by C₆₀ clusters were simulated. The adsorption properties of pure CO₂, the mixtures of CO₂/CH₄, CO₂/N₂ and N₂/O₂ were investigated. The simulated results of the adsorption isotherms, the adsorption quantity, the density fields, the isosteric heats and the selectivity in COF-108s were obtained. It is shown that the impregnation of C₆₀ can enhance the adsorption capacity of CO₂, N₂ and O₂, and the selectivity of CO₂/CH₄, CO₂/N₂ and N₂/O₂ in COF-108. The impregnation of C₆₀ can increase the surface area COF-108 but decrease its free volume and the pore diameter. At low adsorption pressures, the monolayer surface adsorption is dominant. With the increase in adsorption pressure, the dominant factor is changed into the free volume of COF-108 by the multilayer adsorption. The impregnation of C₆₀ plays different roles for the polar or non-polar gases at different pressures.     

Entrapping Molecules in Zeolites Nanocavities: A Thermodynamic and Ab-Initio Study

Adsorption enthalpies of Ar, N₂, CO, H₂O, CH₃CN and NH₃ on H-BEA and H-MFI zeolites and on Silicalite, have been measured calorimetrically at 303K in order to assess the energetic features of dispersive forces interactions (confinement effects), H-bonding interactions with surface silanols and specific interactions with Lewis and Brønsted acidic sites. The adsorption of the molecular probes with model clusters mimicking surface silanols, Lewis and Bronsted sites has been simulated at ab-initio level. The combined use of the two different approaches allowed to discriminate among the different processes contributing to the measured (-Δ_adsH). Whereas CO and N₂ single out contributions from Lewis and Br{\o}nsted acidic sites, Ar is only sensitive to confinement effects. For H₂O, CH₃CN and NH₃ the adsorption on Brønsted sites is competitive with the adsorption on Lewis sites. The energy of interaction of H₂O with all considered zeolites is surprisingly higher than expected on the basis of -Δ_adsH vs PA correlation.     

N-octane diffusivity enhancement via carbon dioxide in silica slit-shaped nanopores – a molecular dynamics simulation

Equilibrium molecular dynamics simulations were conducted to study the competitive adsorption and diffusion of mixtures containing n-octane and carbon dioxide confined in slit-shaped silica pores of width 1.9 nm. Atomic density profiles substantiate strong interactions between CO₂ molecules and the protonated pore walls. Non-monotonic change in n-octane self-diffusion coefficients as a function of CO₂ loading was observed. CO₂ preferential adsorption to the pore surface is likely to attenuate the surface adsorption of n-octane, lower the activation energy for n-octane diffusivity, and consequently enhance n-octane mobility at low CO₂ loading. This observation was confirmed by conducting test simulations for pure n-octane confined in narrower pores. At high CO₂ loading, n-octane diffusivity is hindered by molecular crowding. Thus, n-octane diffusivity displays a maximum. In contrast, within the concentration range considered here, the self-diffusion coefficient predicted for CO₂ exhibits a monotonic increase with loading, which is attributed to a combination of effects including the saturation of the adsorption capacity of the silica surface. Test simulations suggest that the results are strongly dependent on the pore morphology, and in particular on the presence of edges that can preferentially adsorb CO₂ molecules and therefore affect the distribution of these molecules equally on the pore surface, which appears to be required to provide the effective enhancement of n-octane diffusivity.     

Adsorption behaviors of hydrogen sulphide on Au(110) nanoslit array surfaces using molecular dynamics simulations

The adsorption behaviour of gas molecules on detector surfaces has a profound influence on the sensitivity of the detector. For this reason, this study used molecular dynamics simulation to explore the dynamic adsorption behaviour of hydrogen sulphide (H₂S) molecules on various types of Au surfaces, including a planar Au(1?1?0) structure and three types of slit array structures. The influence of system temperature, adsorbate concentration and the slit width of nanoarrays on diffusivity, average adsorption energy and static adsorption amount were systematically examined. Simulation results indicate that the self-diffusivity of the adsorbate molecules increases with temperature but decreases with adsorbate concentration. At low concentrations (~3 mol/L), each type of Au(1?1?0) surface structure shows good capacity to adsorb all H₂S molecules. With increasing concentration at 6.5 mol/L, the high concentration leads to adsorption saturation and many free H₂S molecules in the planar Au(1?1?0) structure. Moreover, desorption also begins to appear on the planar structures at a temperature of 300 K (at 6.5 mol/L). The simulation results indicate that the columnar array structures with a slit width ≥5.76 Å allow molecules to swiftly spread into the slits and provide more stable adsorption sites (i.e. with a higher adsorption energy), which can effectively address the issues of high-temperature desorption and adsorption saturation. Particularly at low temperatures (≤100 K), slit structures presented a level of static adsorption of H₂S that was 30% to 35 higher than that of planar structures.     

Gas adsorption on the Zn-, Pd- and Os-doped armchair (5,5) single-walled carbon nanotubes

The adsorption of NO₂, NH₃, H₂O, CO₂ and H₂ gases on the undoped, Zn–, Pd– and Os–doped armchair (5,5) single–walled carbon nanotubes (SWCNTs) were studied using density functional method. The adsorptions of these five gases on the Zn–, Pd– and Os–doped SWCNTs are obviously stronger than on the undoped SWCNT and their adsorption abilities are in the same order: NO₂ > NH₃ > H₂O > CO₂ > H₂. Adsorption energies for all the studied gases on the undoped, Zn–, Pd– and Os–doped SWCNTs computed at the B3LYP/LanL2DZ level are reported.     

Protein modeling and molecular dynamics simulation of the two novel surfactant proteins SP-G and SP-H

The prospects of a control for a novel gallium nitride pseudo-halide vapor phase epitaxy (PHVPE) with HCN were thoroughly analyzed for hydrocarbons–NH₃–Ga gas phase on the basis of quantum chemical investigation with DFT (B3LYP, B3LYP with D3 empirical correction on dispersion interaction) and ab-initio (CASSCF, coupled clusters, and multireference configuration interaction including MRCI+Q) methods. The computational screening of reactions for different hydrocarbons (CH₄, C₂H₆, C₃H₈, C₂H₄, and C₂H₂) as readily available carbon precursors for HCN formation, potential chemical transport agents, and for controlled carbon doping of deposited GaN was carried out with the B3LYP method in conjunction with basis sets up to aug-cc-pVTZ. The gas phase intermediates for the reactions in the Ga-hydrocarbon systems were predicted at different theory levels. The located π-complexes Ga…C₂H₂ and Ga…C₂H₄ were studied to determine a probable catalytic activity in reactions with NH₃. A limited influence of the carbon-containing atmosphere was exhibited for the carbon doping of GaN crystal in the conventional GaN chemical vapor deposition (CVD) process with hydrocarbons injected in the gas phase. Our results provide a basis for experimental studies of GaN crystal growth with C₂H₄ and C₂H₂ as auxiliary carbon reagents for the Ga-NH₃ and Ga-C-NH₃ CVD systems and prerequisites for reactor design to enhance and control the PHVPE process through the HCN synthesis.     

Nanoconfined gases,liquids and liquid crystals in porous materials

We propose here to give an overview of gases and liquids adsorption in the materials of Institute Lavoisier (MIL)-101(Cr), MIL-53(Cr) and silica materials. We present some recent results of systems of interests such as the H₂ adsorption in MIL-101(Cr) and CO₂ and H₂S adsorption in the MIL-53(Cr) material. In addition, we will examine the sensitivity in water force field for water adsorption in hydrophilic and hydrophobic silica nanopores and we evaluate the Gay–Berne liquid crystal adsorption in the smooth and rough pores.     

THE PERMEABILITY OF THIN DRY COLLODION MEMBRANES

Dry thin collodion membranes have been prepared which are permeable to water, ammonia, weak acids of low molecular weight, HCl gas, O₂, CO₂, and H₂S, but are impermeable to strong electrolytes and substances of high molecular weight. The permeability to gases does not depend on the density, so that the gases do not pass through pores in the membrane.     

Investigation on the mechanism of H<Subscript>2</Subscript>S removal by biological activated carbon in a horizontal biotrickling filter

The use of supporting media for the immobilization of microorganisms is widely known to provide a surface for microbial growth and a shelter that protects the microorganisms from inhibitory compounds. In our previous studies, activated carbon (AC) alone used as a support medium for H₂S biological removal was proved prompt and efficient in a bench-scale biofilter and biotrickling filter. In this study, the mechanisms of H₂S elimination using microbial immobilized activated carbon, i.e., biological activated carbon (BAC), are investigated. A series of BAC as supporting medium were taken from the inlet to outlet of a bench-scale horizontal biotrickling filter to examine the different effects of physical/chemical adsorption and microbial degradation on the overall removal of H₂S. The surface properties of BAC together with virgin and exhausted carbon (after H₂S breakthrough test, non-microbial immobilization) were characterized using the sorption of nitrogen (Braunner–Emmett–Teller test), scanning electron microscopy (SEM), surface pH, thermal, carbon–hydrogen–nitrogen–sulfur (CHNS) elemental and Fourier transform infrared (FTIR) analyses. Tests of porosity and surface area provide detailed information about the pore structure of BAC along the bed facilitating the understanding of potential pore blockages due to biofilm coating. A correlation between the available surface area and pore volume with the extent of microbial immobilization and H₂S uptake is evidenced. SEM photographs show the direct carbon structure and biofilm coated on carbon surface. FTIR spectra, differential thermogravimetric curves and CHNS results indicate less diversity of H₂S oxidation products on BAC than those previously observed on exhausted carbon from H₂S adsorption only. The predominant oxidation product on BAC is sulfuric acid, and biofilm is believed to enhance the oxidation of H₂S on carbon surface. The combination of biodegradation and physical adsorption of using BAC in removal of H₂S could lead to a long-term (i.e., years) good performance of biotrickling filters and biofilters based on BAC compared to carbon adsorption only.     

Molecular simulation of shale gas adsorption and diffusion in inorganic nanopores

We studied the structural and dynamical properties of methane and ethane in montmorillonite (MMT) slit pore of sizes 10, 20 and 30 Å using grand canonical Monte Carlo and classical molecular dynamics (MD) simulations. The isotherm, at 298.15 K, is generated for pressures up to 60 bar. The molecules preferentially adsorb at the surface as indicated by the density profile. In case of methane, we observe only a single layer, at the pore wall, whose density increases with increasing pressure. However, ethane also displays a second layer, though of low density in case of pore widths 20 and 30 Å. In-plane self-diffusion coefficient, D_‖, of methane and ethane is of the order of 10^? 6 m²/s. At low pressure, D_‖ increases significantly with the pore size. However, D_‖ decreases rapidly with increasing pressure. Furthermore, the effect of pore size on D_‖ diminishes at high pressure. Ideal adsorbed solution theory is used to understand the adsorption behaviour of the binary mixture of methane (80%) and ethane (20%) at 298.15 K. Furthermore, we calculate the selectivity of the gases at various pressures of the mixture, and found high selectivity for ethane in MMT pores. However, selectivity of ethane decreases with increase in pressure or pore size.     

Theoretical Study of the TiO2 and MgO Surface Acidity and the Adsorption of Acids and Bases

We present ab-initio periodic Hartree–Fock calculations (crystal program) of small molecules on TiO₂ and MgO. The adsorption of the molecules may be molecular or dissociative. This depends on their acid and basic properties in the gas phase. For the molecular adsorption, the molecules are adsorbed as bases on Ti(+IV) sites, the adsorption energies correlate with the proton affinities. The dissociations on the surface correlate with the gas phase cleavages: thus, the dissociation of MeOH leads to a preferential basic cleavage (the fragment HO– is adsorbed on a Ti⁺⁴ ion and the fragment Me⁺ is adsorbed on a O²– ion of the oxide). The opposite result is obtained with MeSH. Another important factor is the adsorbate–adsorbate interaction: favorable cases are a sequence of H-bonds for the hydroxyl groups resulting from the water dissociation and the mode of adsorption for the ammonium ions. Lateral interactions also force the adsorbed CO₂ molecules to bend over the surface so that their mutual orientation resembles the geometry of the CO₂ dimer. With respect to water adsorption, MgO appears to be a basic oxide. As experimentally observed, NH₃ adsorbs preferentially on TiO₂ and CO₂ on MgO. However, this difference of reactivity should not be expressed in terms of acid vs. basic behaviour but in terms of hard and soft acidity. The MgO surface is a 'soft' acidic surface that reacts preferentially with the soft base, CO₂.     

Polymers of intrinsic microporosity for gas permeation: a molecular simulation study

We report a molecular simulation study for gas permeation in two membranes constructed from polymers of intrinsic microporosity (PIM-1 and PIM-7). With rigid ladder polymer chains, the membranes posses approximately 47.7 and 46.6% fractional free volumes (FFVs) in PIM-1 and PIM-7, respectively. The voids in the membranes have a diameter up to 9 Å and are largely interconnected. The sorption and diffusion of four gases (H₂, O₂, CH₄ and CO₂) were calculated by Monte Carlo and molecular dynamics simulations. The solubility coefficients increase in the order of H₂ < O₂ < CH₄ < CO₂, while the diffusion coefficients increase in the following order: CH₄ < CO₂ < O₂ < H₂. The simulation results agree well with experimental data, particularly for the solubility coefficients. The solubility and diffusion coefficients correlate well separately with the critical temperatures and effective diameters of gases. These molecular-based correlations can be used in the prediction for other gases. As attributed to the microporous structure, PIM-1 and PIM-7 outperform most glassy polymeric membranes in sorption and diffusion. PIM-1 has larger solubility and diffusion coefficients than PIM-7 because the cyano groups in PIM-1 lead to a stronger affinity and a larger FFV. The simulated solubility, diffusivity and permeation selectivities of CO₂/H₂, CO₂/O₂ and CO₂/CH₄ are consistent with experimental data. The quantitative microscopic understanding of gas permeation in the PIM membranes is useful for the new development of high-performance membranes.     

Biogas upgrading using single-walled carbon nanotubes by molecular simulation

Abstract

Biogas from anaerobic digestion of biological wastes is a renewable energy resource that mainly contains CH₄, CO₂, trace amounts of H₂S and a fraction of H₂O vapour. In order to transfer biogas into biomethane to meet the standards for use as vehicle fuel or for injection in the natural gas grid, removing H₂S from biogas in advance is necessary. In addition, biogas is usually saturated with water vapour. It is significant to study the effect of the presence of H₂O on the biogas separation performance. Adsorption of H₂S/CO₂/CH₄ and H₂O/CO₂/CH₄ ternary mixtures using single-walled carbon nanotubes (SWCNT) were investigated via the Grand Canonical Monte Carlo (GCMC) method. We studied the effects of carbon nanotube diameter, –COOH modification, temperature and pressure on H₂S adsorption. The results indicate that the presence of hydrophilic –COOH groups does affect the separation of H₂S/CO₂/CH₄ mixtures. Temperature swing adsorption is more suitable than pressure swing adsorption for the separation of H₂S/CO₂/CH₄ mixtures. The effect of water vapour on the separation of CO₂/CH₄ was also investigated. The result shows that the presence of H₂O has little effect on the selectivity of CO₂/CH₄ in pristine CNT, but the selectivity of CO₂/CH₄ with the presence of H₂O is markedly enhanced after modification in –COOH modified SWCNT with specific modification degree. It is expected that this work could provide some useful information for biogas upgrading.     

Resistance of citrus fruit to mass transport of water vapor and other gases

The resistance of oranges (Citrus sinensis L. Osbeck) and grapefruit (Citrus paradisi Macf.) to ethylene, O₂, CO₂, and H₂O mass transport was investigated anatomically with scanning electron microscope and physiologically by gas exchange measurements at steady state. The resistance of untreated fruit to water vapor is far less than to ethylene, CO₂ and O₂. Waxing partially or completely plugs stomatal pores and forms an intermittent cracked layer over the surface of fruit, restricting transport of ethylene, O₂, and CO₂, but not of water; whereas individual sealing of fruit with high density polyethylene films reduces water transport by 90% without substantially inhibiting gas exchange.

Stomata of harvested citrus fruits are essentially closed. However, ethylene, O₂ and CO₂ still diffuse mainly through the residual stomatal opening where the relative transport resistance (approximately 6,000 seconds per centimeter) depends on the relative diffusivity of each gas in air. Water moves preferentially by a different pathway, probably through a liquid aqueous phase in the cuticle where water conductance is 60-fold greater. Other gases are constrained from using this pathway because their diffusivity in liquid water is 10⁴-fold less than in air.

     

Editorial

Abstract

Grand canonical molecular dynamics (GCMD) simulations are used to study the adsorption and desorption of Lennard-Jones nitrogen in three slit pore junction models of microporous graphite. These networks consist of two narrow pores separated by a wider (cavity) pore. We report results for cases where the narrow pore has a width of only two or three molecular diameters. Using the GCMD technique, a novel freezing transition is observed which results in pore blocking in the narrow pores of the network, which are less than 1 nm wide. This freezing results from the adsorption energy barrier at the junction between the narrow and wider pores. This type of pore blocking could account for the apparent increase in pore volume with increasing temperature that has been experimentally observed in microporous graphite systems. For networks in which the narrower pores are somewhat larger, with a width of 1.28 nm, this pore blocking effect is much reduced, and adsorbate molecules enter and fill the central cavity. In such cases, however, desorption is incomplete, some residual adsorbate remaining in the central cavity even at the lowest pressures.     

Nonallergenic urushiol derivatives inhibit the oxidation of unilamellar vesicles and of rat plasma induced by various radical generators

Urushiols consist of an o-dihydroxybenzene (catechol) structure and an alkyl chain of 15 or 17 carbons in the 3-position of a benzene ring and are allergens found in the family Anacardiaceae. We synthesized various veratrole (1,2-dimethoxybenzene)-type and catechol-type urushiol derivatives that contained alkyl chains of various carbon atom lengths, including –H, –C₁H₃, –C₅H₁₁, –C₁₀H₂₁, –C₁₅H₃₁, and –C₂₀H₄₁, and investigated their contact hypersensitivities and antioxidative activities. 3-Decylcatechol and 3-pentadecylcatechol displayed contact hypersensitivity, but the other compounds did not induce an allergic reaction, when the ears of rats were sensitized by treatment with the compounds every day for 20 days. Catechol-type urushiol derivatives (CTUDs) exerted very high radical-scavenging activity on the 1,1-diphenyl-2-picrylhydrazyl radical and inhibited lipid peroxidation in a methyl linoleate solution induced by 2,2′-azobis(2,4-dimethylvaleronitrile) (AMVN). However, veratrole-type urushiol derivatives did not scavenge or inhibit lipid peroxidation. CTUDs also acted as effective inhibitors of lipid peroxidation of the egg yolk phosphatidylcholine large unilamellar vesicle (PC LUV) liposome system induced by various radical generators such as AMVN, 2,2′-azobis(2-amidino-propane) dihydrochloride, and copper ions, although their efficiencies differed slightly. In addition, CTUDs suppressed formation of cholesteryl ester hydroperoxides in rat blood plasma induced with copper ions. CTUDs containing more than five carbon atoms in the alkyl chain showed excellent lipophilicity in a n-octanol/water partition experiment. These compounds also exhibited high affinities to the liposome membrane using the ultrafiltration method of the PC LUV liposome system. Therefore, CTUDs seem to act as efficient antioxidative compounds against membranous lipid peroxidation owing to their localization in the phospholipid bilayer. These results suggest that nonallergenic CTUDs act as antioxidants to protect against oxidative damage of cellular and subcellular membranes.     

Energy gaps of graphene clusters: the first-principles calculations based on high-throughput screening

Abstract

With the enumeration of the triangular lattice fragments, we have systematically investigated the graphene clusters (C_nH_m, n = 14 – 24) with various sizes and shapes, whose structural stabilities and electronic properties are studied by the Hückel molecular orbital (HMO) method and the first-principles calculation. According to the formation energies, we show the structural stabilities of the clusters are closely related to the shape and size, as well as the chemical potential of hydrogen. The energy gaps obtained from the HMO method are in the same trend with the ones calculated by the first-principles calculations, indicating the effective screening of the gap minimum and maximum in a fast speed. There is a general decreasing of the energy gaps with the size increment due to the quantum confinement, meanwhile, the gaps are also highly dependent on the shape of the clusters for those with the same number of carbon atom.     

Use of activated carbon as a support medium for H<Subscript>2</Subscript>S biofiltration and effect of bacterial immobilization on available pore surface

The use of support media for the immobilization of microorganisms is widely known to provide a surface for microbial growth and a shelter that protects the microorganisms from inhibitory compounds. In this study, activated carbon is used as a support medium for the immobilization of microorganisms enriched from municipal sewage activated sludge to remove gas-phase hydrogen sulfide (H₂S), a major odorous component of waste gas from sewage treatment plants. A series of designed experiments is used to examine the effect on bacteria-immobilized activated carbon (termed biocarbon) due to physical adsorption, chemical reaction, and microbial degradation in the overall removal of H₂S. H₂S breakthrough tests are conducted with various samples, including microbe-immobilized carbon and Teflon discs, salts-medium-washed carbon, and ultra-pure water-washed carbon. The results show a higher removal capacity for the microbe-immobilized activated carbon compared with the activated carbon control in a batch biofilter column. The increase in removal capacity is attributed to the role played by the immobilized microorganisms in metabolizing adsorbed sulfur and sulfur compounds on the biocarbon, hence releasing the adsorption sites for further H₂S uptake. The advantage for activated carbon serving as the support medium is to adsorb a high initial concentration of substrate and progressively release this for microbial degradation, hence acting as a buffer for the microorganisms. Results obtained from surface area and pore size distribution analyses of the biocarbon show a correlation between the available surface area and pore volume with the extent of microbial immobilization and H₂S uptake. The depletion of surface area and pore volume is seen as one of the factors which cause the onset of column breakthrough. Microbial growth retardation is due to the accumulation of metabolic products (i.e., sulfuric acid); and a lack of water and nutrient salts in the batch biofilter are other possible causes of column breakthrough.