The Adsorption of Argon and Nitrogen in Silicalite-1 Zeolite: A Grand Canonical Monte-Carlo study

Abstract

Grand Ensemble Monte-Carlo simulations of adsorption of argon and nitrogen in silicalite have been performed using a new adsorbate/zeolite potential function. In both cases, a good agreement with zero coverage data (Henry law constant and isosteric heat of adsorption) has been obtained. For argon, the simulated isotherm at 77 K exhibits the experimentally observed step. This is attributed to an in site/off-site phase transition of the adsorbed phase. The calculated neutron diffraction spectra are in reasonable agreement with those obtained experimentally. Furthermore, we suggest, in light of recent ⁴⁰Ar diffraction experiments of Tosi-Pellenq and Coulomb [18,44], that the shift in pressure between the simulated and the experimental isotherms corresponds to changes in the zeolite structure accompanied with the adsorbate phase transition itself. For nitrogen, only the first of the two experimentally observed steps is reproduced in the simulation. This step corresponds to an ordering of the adsorbed phase. The fact that the second step is missing in the simulated isotherm supports the hypothesis of a distortion of the zeolite framework under the stress of the adsorbed fluid at high loading.     

Adsorption of Binary Mixtures in a Zeolite Micropore

Abstract

We investigate the selective adsorption of xenon, argon, and methane in zeolite NaA by applying the grand canonical ensemble Monte Carlo simulation technique to an adsorbed binary mixture and to two reference systems: i) an adsorbed single component system and ii) a bulk mixture. We define and calculate selectivities and excess densities due to i) mixing and ii) adsorption in terms of differences between the binary adsorbed system and these reference systems. We observe that xenon selectively adsorbs in both xenon-argon and xenon-methane mixtures at low chemical potential (low pressure) due to its greater energetic interaction with the zeolite. However, a reversal in selectivity occurs at higher chemical potential in both of these mixtures. This is due in large part to the greater efficiency in which the smaller component “packs” in the pore as compared to the bulk. We show that the crossover in selectivity occurs at a lower chemical potential for a mixture where one component can occupy regions of the porespace inaccessible to the other. We suggest that this crossover in selectivity may be a general feature of microporous adsorption.     

Modelling Diffusion in Zeolites by Molecular Dynamics Simulations

Abstract

The diffusion of molecules sorbed in zeolites is of growing interest for understanding the mechanisms of chemical processes with regard to selectivity and reactivity [1].

MD simulations give insight into physical systems on the molecular level allowing to study and visualize the motion of molecules even beyond the possibilities of experiments [2,3]. Single system parameters can easily be varied to study their influence, also those parameters that are fixed in reality (e.g., the size of particles). We present a cross section of our recent work to illustrate the capabilities of MD: The self diffusion coefficients (D) of a mixture of methane and xenon in silicalite show remarkable deviations from those of the pure species. This is shown and confirmed by PFG NMR experiments [4].

Simulating ethane in zeolite A the mechanism of diffusion has been studied. The effects of rotation on the diffusion lead to cases where D decreases with growing temperature [5].

The independence of self diffusion on lattice vibrations is proven even for zeolites with windows of guest particle size comparing simulations with rigid and vibrating zeolite lattice [6].     

Production of NO and N2O by the Heterotrophic Nitrifier Alcaligenes faecalis parafaecalis under Varying Conditions of Oxygen Saturation

Production of NO and N ₂ O by the heterotrophic nitrifier Alcaligenes faecalis subsp. parafaecalis was studied during growth in batch and continuous culture on peptone-meat extract medium. Depending on oxygen saturation level, medium redox status and amount of substrate supplied, the microorganisms produced 0.002–0.25 mg NO-N h^{- 1} (g protein)^{- 1} and 0.16–2.4 mg N ₂ O-N h^{- 1} (g protein)^{- 1} . Maximum rates of nitrogen oxides production were observed during peak events initiated by sudden changes of oxygen supply in the medium and were due to combined nitrification/denitrification taking place simultaneously within the cells. Based on model simulations of enzymatic kinetics of denitrification, possible mechanisms of increased nitrogen oxides production during periods of changes in oxygen supply are suggested.     

Physicochemical selectivity for cations and CEC of grass roots

Summary A study of root selective adsorption of Ca, Mg, K and Na was made with rootlets of 20 species of Gramineae (from natural vegetation) exhibiting low cation exchange capacities (CEC) scattered over a narrow range 7.6–16.7 meq 100 g^-1 dry matter). For each sample of rootlets successive measurements were made of its CEC (amount of Ca adsorbed from 0.05 N calcium nitrate solution) and the amounts of Ca, Mg, K and Na adsorbed from a tetracationic nitrate solution having equal concentrations (0.05 N) of each cation. Rootlet selectivity for bivalent cations increased with CEC and was greater for Ca than for Mg. In contrast, the selectivity for monovalent cations increased with decreasing CEC and was greater for K than for Na. A root's selective adsorption may therefore be governed by its CEC, in conformation with the exchange laws, as is the case with clays and resins. Paper read at the XII International Botanical Congress (mineral nutrition session) held in Leningrad, July, 1975.     

Surface modified zeolite, a novel carrier material for Azotobacter chroococcum

A new immobilization matrix based on zeolite has been developed to immobilize Azotobacter chroococcum, for fixing nitrogen, with an intention to hold the cells in the root zone of the plants and to protect them under stressful conditions. The matrix has been developed by modifying the surface of the zeolite with surfactant. This enhances the hydrophobicity of the material and also modifies the surface charge, which in turn enhances the immobilization. Surface modified zeolite-A (SMZ-A) has been compared with commercial zeolite-A (CZA) for immobilization efficiency. CZA is non-toxic for A. chroococcum but is inefficient to adsorb the cells whereas SMZ-A showed 100% adsorption of the microbial cells wherein it was observed that for 1 l of broth culture with total viable count of 10⁸ cfu ml⁻¹ cells of A. chroococcum, a minimum dose of 0.7 g SMZ-A and minimum contact time of 10 h is required to achieve 100% adsorption. Adsorption was confirmed by the cell count and light as well as scanning electron microscopy. Most importantly, the cells adsorbed on SMZ-A could fix the atmospheric nitrogen up to 13 mg g⁻¹ sucrose consumed, which was comparable with the control (unadsorbed cells), which confirms the survival and nitrogen fixation activity of the bacteria. Responsible Editor: Euan K. James.     

Zeolite adsorption site location and shape shown by simulated isodensity surfaces

The graphics software package Ribbons¹ is used to display isodensity surfaces of Xe atoms adsorbed in the alpha cage of zeolite NaA. The location, size, and shape of the adsorption sites are highly dependent on the loading and the crystal cation content. When the zeolite has a high number of cations, ellipsoidal sites arrange in a cuboctahedron. When the zeolite has fewer cations, cone-shaped sites arrange in an octahedron at low loading, but at high loading the sites become ellipsoidal and new sites form at cuboctahedral positions. The effect of the nature of the adsorption site on the development of a universal adsorption model is discussed.     

Effect of anoxic conditions on wood-decay fungi treated with argon or nitrogen

The effects of low-oxygen conditions, achieved with either argon or nitrogen gas, on the viability of wood-decay fungi Coniophora puteana and Antrodia vaillantii, grown on artificial growth medium, were tested. Initial tests for viability were run after 1, 2, 3 and 4 weeks of exposure to low oxygen conditions, at oxygen levels in the vessels maintained below 10 ppm. Treatment was later extended to 10 and 16 weeks. Tests included measurements of CO₂ production and determination of the ability of fungal tissue to regenerate. The effect of anoxic conditions on the mycelia of treated fungal species was expressed as an increased time needed for regeneration or as a complete absence of growth of inocula taken from the exposed cultures. The cultures that were retarded by the low-oxygen environment consequently produced less CO₂ per hour. For C. puteana cultures, the effects of anoxic treatment became apparent in the second week of the treatment. The number of affected cultures rose steadily with the prolongation of anoxic treatment. By the 16th week of the experiment, 80% of the inocula of C. puteana did not regenerate. A. vaillantii inocula regeneration was not affected until after the fourth week of treatment. The influence of anoxic treatment on the cultures of this species was more pronounced in the test on the 10th and especially on the 16th week, when 67% of inocula did not regenerate. Argon or nitrogen gas caused the same degree of loss of viability in both the fungal species tested. In general, A. vaillantii mycelial cultures proved to be less sensitive to anoxic conditions, caused by either argon and nitrogen gas.     

Computer Simulation of Isothermal Mass Transport in Graphite Slit Pores

Abstract

Results are presented from a simulation study of the mass transport of oxygen and nitrogen through graphite slit pores. The work is motivated by an attempt to understand the molecular origins of the kinetic selectivity displayed when air is separated into its major components using pressure swing adsorption. A combination of non-equilibrium molecular dynamics (NEMD), equilibrium molecular dynamics (EMD) and grand canonical Monte Carlo methods has been employed in our study to extract the maximum information. Transport diffusivities, self-diffusivities, permeabilities and Darken thermodynamic factors have been calculated as a function of pore width and temperature for pure component oxygen and nitrogen. In addition, new EMD simulation data for an 80:20 mixture of nitrogen and oxygen is reported, including a direct calculation of the Stefan-Maxwell coefficients. The results are discussed in terms of the oxygen selectivity and the possible mechanisms, which increase or decrease this quantity.

We find that the pore width behaviour of the diffusion coefficients consists of three distinct regimes: a regime at larger pore widths in which single component diffusion coefficients are largely independent of pore width, an optimum pore width at which both diffusivities increase substantially but the slit pore is selective towards nitrogen, and a regime at very low pore widths at which the diffusivities decrease sharply, but the slits are selective towards oxygen. The mechanism behind each of these regimes is discussed in terms of “entropic” effects and potential barrier heights.

We have also found that permeability selectivity is substantially reduced in a mixture of the two gases with a composition similar to that of air. Cross diffusion coefficients in the mixture have been calculated and shown to be non-negligible.     

SOME PROPERTIES OF SUSPENDED SEDIMENT ADSORBED CATIONS IN TURBID FRESHWATERS OF SOUTH AFRICA

SUMMARY

Large quantities of suspended sediments are common in many of South Africa's fresh-waters. Temporal and spatial variations in tile amounts of cations adsorbed were recorded. The adsorption appears to be dependent on valency, because greater quantities of the higher valencies are adsorbed. Ca⁺⁺ dominated the adsorbed cations and Mg the dissolved fraction. Water originating from the Beaufort Series contained high sodium concentrations. Fe⁺⁺ dominated the adsorbed minor cations. Large quantities of sediments transported by rivers enter impoundments. The adsorbed ions transported in this way are influenced by the type of suspended sediment and form a significant part of the total input of ions.     

Influence of the reaction medium on the product distribution of peroxidase-catalysed oxidation of p-cresol

Summary p-Cresol was oxidized by hydrogen peroxide in a reaction catalysed by horseradish peroxidase and the low molecular weight products were investigated. In aqueous media Pummerer's ketone (I) was the dominating product but in organic media the product distribution was quite different; 2,2'-dihydroxy-5,5'-dimethyldiphenyl (II) was the main low molecular weight product. Similar product distributions were obtained with peroxidase adsorbed on a solid support and suspended in toluene and with peroxidase solubilized in a microemulsion containing the same solvent. The best selectivity for the formation of (II) was obtained when the enzyme was adsorbed on Celite and suspended in water-saturated chloroform with 0.5% (v/v) extra water added. The yield of low molecular weight products in this case was 28%; of this fraction, 95% was (II). Offprint requests to: P. Adlercreutz     

Cation redistribution upon dehydration of Na58Y faujasite zeolite: a joint neutron diffraction and molecular simulation study

Using neutron scattering and Monte Carlo simulation, we investigate the distribution of cations in Na₅₈Y faujasite upon (de)hydration. We introduce a new method for the assignment of cations to specific sites in molecular simulations from their local environment. This allows us to bypass the need of the coordinates of crystallographic sites, which vary as water adsorption induces changes in the zeolite framework structure. Although the agreement between experiments and simulation is excellent at high temperature, some differences are observed below 150°C. We show that these differences are due to the presence of water and that temperature itself as well as adsorption-induced deformation of the framework play a less important role. We demonstrate the migration of sodium to sites III upon water adsorption, not observed for other Si:Al ratios.     

Physiological studies on nitrogen fixation in the blue-green alga anabaena cylindrica

Summary Short-term manometric experiments with bacteria-free cultures of Anabaena cylindrica showed that the close dependency of nitrogen fixation upon photosynthesis could be temporarily eliminated in nitrogen-starved cells. Initial rates of nitrogen uptake by these cells in the absence of carbon dioxide were equally rapid in the light and dark, decreasing and finally ceasing after two hours. Continued steady nitrogen uptake was only maintained for long periods in the presence of carbon dioxide in the light. In the dark, nitrogen uptake was accompanied by carbon dioxide evolution.More oxygen was evolved in the light by cells fixing nitrogen than by those incubated under argon. This additional oxygen evolution could be accounted for by extra carbon dioxide fixation in the presence of nitrogen.Of a number of organic compounds tested, only sodium pyruvate stimulated nitrogen fixation. This stimulation was achieved both in the light and dark and in the presence and absence of carbon dioxide, showing that the role of pyruvate was other than acting as a carbon skeleton.Three metabolic inhibitors, cyanide and chlorpromazine (chiefly respiratory) and phenylurethane (photosynthetic) differentially inhibited photosynthesis and nitrogen fixation. The latter inhibitor had a more marked effect on photosynthesis while the two chiefly respiratory inhibitors had a stronger effect on nitrogen fixation.     

Effect of Pt and Sn on the adsorption of n-heptane in γ-Al2O3 catalyst models

The Grand Canonical Monte-Carlo (GCMC) method has been used to carry out simulations of the adsorption of n-heptane in models of naphtha-reforming catalysts. Models used in the study differed in the number and distribution of metal atoms—Pt and Sn. The number of adsorbed n-heptane molecules grows linearly with increasing number of metal atoms. The effect of Pt content on the adsorption of n-heptane molecules is most distinct at approximately 100 kPa and within the lower range of the temperatures investigated. In the models of bimetallic catalysts, the effect of the two metals is additive.Figure Effect of Pt and Sn on number of n-heptane molecules adsorbed in Al₂O₃ catalyst in 773 K and 1000 kPa.      

Electron-donating para-methoxy converts a benzamide-isoquinoline derivative into a highly Sigma-2 receptor selective ligand

The sigma-2 (σ2) receptor has been suggested to be a promising target for pharmacological interventions to curb tumor progression. Development of σ2-specific ligands, however, has been hindered by lack of understanding of molecular determinants that underlie selective ligand-σ2 interactions. Here we have explored effects of electron donating and withdrawing groups on ligand selectivity for the σ2 versus σ1 receptor using new benzamide-isoquinoline derivatives. The electron-donating methoxy group increased but the electron-withdrawing nitro group decreased σ2 affinity. In particular, an extra methoxy added to the para-position (5e) of the benzamide phenyl ring of 5f dramatically improved (631 fold) the σ2 selectivity relative to the σ1 receptor. This para-position provided a sensitive site for effective manipulation of the sigma receptor subtype selectivity using either the methoxy or nitro substituent. Our study provides a useful guide for further improving the σ2-over-σ1 selectivity of new ligands.     

Excited-state equilibration in a meso-/microporous material-hosted bichromophoric [Ruthenium (2,2′-bipyridine)3]: Reversible energy transfer and photosensitized electron pumping

A bichromophoric pyrene-appended [Ru(bpy)₃]²⁺-type complex (bpy = 2,2′-bipyridine) showing rapid, reversible intramolecular energy transfer processes leading to an excited-state equilibration in homogeneous solutions was introduced into different photoinert supports, namely MCM-41, ethylene-bridged periodic mesoporous organosilica (PMO), and zeolite NaY. Its photophysical behaviour in each of these supports is compared with its behaviour in solution and with that of an appropriate model lacking the pyrene chromophore, [Ru(bpy)₂dmb]²⁺. Results suggest that the excited-state equilibration process which is operational in homogeneous solutions leading to long-luminescence lifetimes, is equally observed in all the different supports. A diminished oxygen sensitivity and prolonged luminescence lifetime was recorded for all the complexes included with respect to the analogous species in solution. The adsorbed pyrene-appended [Ru(bpy)₃]²⁺ complex is also shown to participate in photosensitized electron pumping from zeolite NaY to a size-excluded methylviologen in solution more efficiently than the adsorbed parent complex.     

Photoproduction of Hydrogen by the Marine Heterocystous Cyanobacterium Anabaena Species TU37-1 Under a Nitrogen Atmosphere

Hydrogen production rates by Anabaena sp. strain TU37-1 obtained after an initial 1-day incubation period were approximately 70 to 80 and 3 to 9 µmol (mg chl)^–1 h^–1 under argon and nitrogen atmospheres, respectively. Hydrogen production under argon was not enhanced by addition of carbon dioxide, but was enhanced to some extent under nitrogen by increasing the initial carbon dioxide concentration. Rates of hydrogen and oxygen production during the initial 7-hour period were 15 and 220 µmol (mg chl)^–1 h^–1, respectively, in vessels with 18.5% initial carbon dioxide. Hydrogen production under nitrogen was enhanced by addition of carbon monoxide (1%). The rate obtained from the initial 1-day incubation period was about 40 µmol (mg chl)^–1 h^–1, which corresponded to about 60% of that under argon. On the basis of these observations, a possible strategy for hydrogen production by nitrogen-fixing cyanobacteria under nitrogen in the presence of carbon monoxide is indicated.     

Antimicrobial action and efficiency of silver-loaded zeolite X

Aims: To synthesize silver-loaded zeolite X and establish the extent to which it persist in its antimicrobial action against strains of Escherichia coli K12W-T, Pseudomonas aeruginosa NCIMB8295 and Staphylococcus aureus NCIMB6571. Methods and Results: The antimicrobial action and efficacy of silver-loaded zeolite X on Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa were investigated. Zeolite X was synthesized and loaded with Ag⁺ by ion exchange. This resulted in 2·0% (w/w) loading of Ag⁺ in the zeolite framework and 5·8% (w/w) on the zeolite. Escherichia coli and Pseudomonas aeruginosa and Staphylococcus aureus suspended in tryptone soya broth were exposed to 0·15, 0·25, 0·5 or 1·0 g l⁻¹ of silver-loaded zeolite X for a period up to 24 h. No viable cells were detected for any of the three micro-organisms within 1 h. Silver-loaded zeolite X, retrieved three times from the first exposure cultures, was washed with de-ionized water and added to fresh bacterial suspensions. The results showed that the silver-loaded zeolite X retained its antimicrobial action. Conclusions: Silver-loaded zeolite X persisted in its antimicrobial action against all three micro-organisms. Significance and Impact of the study: The results are significant for the longevity of antimicrobial action of silver-loaded zeolite X.     

Insights into discharge argon-mediated biofilm inactivation

Formation of bacterial biofilms at solid–liquid interfaces creates numerous problems in biomedical sciences. Conventional sterilization and decontamination methods are not suitable for new and more sophisticated biomaterials. In this paper, the efficiency and effectiveness of gas discharges in the inactivation and removal of biofilms on biomaterials were studied. It was found that although discharge oxygen, nitrogen and argon all demonstrated excellent antibacterial and antibiofilm activity, gases with distinct chemical/physical properties underwent different mechanisms of action. Discharge oxygen- and nitrogen-mediated decontamination was associated with strong etching effects, which can cause live bacteria to relocate thus spreading contamination. On the contrary, although discharge argon at low powers maintained excellent antibacterial ability, it had negligible etching effects. Based on these results, an effective decontamination approach using discharge argon was established in which bacteria and biofilms were killed in situ and then removed from the contaminated biomaterials. This novel procedure is applicable for a wide range of biomaterials and biomedical devices in an in vivo and clinical setting.