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.     

The role of molecular modelling and simulation in the discovery and deployment of metal-organic frameworks for gas storage and separation*

ABSTRACT

Metal-organic frameworks (MOFs) are highly tuneable, extended-network, crystalline, nanoporous materials with applications in gas storage, separations, and sensing. We review how molecular models and simulations of gas adsorption in MOFs have informed the discovery of performant MOFs for methane, hydrogen, and oxygen storage, xenon, carbon dioxide, and chemical warfare agent capture, and xylene enrichment. Particularly, we highlight how large, open databases of MOF crystal structures, post-processed to enable molecular simulations, are a platform for computational materials discovery. We discuss how to orient research efforts to routinise the computational discovery of MOFs for adsorption-based engineering applications.     

Healthcare Professionals’ and Policy Makers’ Views on Implementing a Clinical Practice Guideline of Hypertension Management: A Qualitative Study

IntroductionMost studies have reported barriers to guideline usage mainly from doctors’ perspective; few have reported the perspective of other stakeholders. This study aimed to determine the views and barriers to adherence of a national clinical practice guideline (CPG) on management of hypertension from the perspectives of policymakers, doctors and allied healthcare professionals.MethodsThis study used a qualitative approach with purposive sampling. Seven in depth interviews and six focus group discussions were conducted with 35 healthcare professionals (policy makers, doctors, pharmacists and nurses) at a teaching hospital in Kuala Lumpur, Malaysia, between February and June 2013. All interviews were audio-recorded, transcribed verbatim and checked. Thematic approach was used to analyse the data.ResultsTwo main themes and three sub-themes emerged from this study. The main themes were (1) variation in the use of CPG and (2) barriers to adherence to CPG. The three sub-themes for barriers were issues inherent to the CPG, systems and policy that is not supportive of CPG use, and attitudes and behaviour of stakeholders. The main users of the CPG were the primary care doctors. Pharmacists only partially use the guidelines, while nurses and policy makers were not using the CPG at all. Participants had suggested few strategies to improve usage and adherence to CPG. First, update the CPG regularly and keep its content simple with specific sections for allied health workers. Second, use technology to facilitate CPG accessibility and provide protected time for implementation of CPG recommendations. Third, incorporate local CPG in professional training, link CPG adherence to key performance indicators and provide incentives for its use.ConclusionsBarriers to the use of CPG hypertension management span across all stakeholders. The development and implementation of CPG focused mainly on doctors with lack of involvement of other healthcare stakeholders. Guidelines should be made simple, current, reliable, accessible, inclusive of all stakeholders and with good policy support.     

不同郁闭度控制下马尾松（Pinus massoniana）人工林土壤动物群落特征

采用郁闭度控制方法,形成马尾松人工林郁闭度梯度(0.5,0.6,0.7,0.8,0.9)试验样地,研究不同郁闭度下土壤动物群落特征。运用手捡法、Tullgren法和Baermann法进行土壤动物群落调查。结果显示,1)共捕获土壤动物8 860只,隶属于4门11纲25目111科,其中大型土壤动物589只,以蜚蠊、疣和康叭为优势类群;中小型土壤动物8271只,以等节和丽甲螨为优势类群。2)大型土壤动物以杂食性为主,杂食性在0.9郁闭度显著增加,捕食性显著减少;中小型土壤动物以菌食和腐食性为主,腐食性土壤动物随郁闭度增大逐渐增加,菌食性逐渐减少。中小型捕食性土壤动物在郁闭度0.5—0.7显著增加,0.7—0.9无显著变化。3)大型土壤动物仅5—10 cm土层类群数在0.9郁闭度显著增加;中小型土壤动物总个体数及枯落物层个体数、5—10 cm土层类群数显著增加(0.5—0.7)后减少(0.7—0.9);中小型土壤动物总类群数在郁闭度0.5—0.6显著增加,枯落层类群数在郁闭度0.5—0.8显著增加,0.8—0.9显著减少。4)大型、中小型土壤动物多样性在土层中差异显著。中小型土壤动物Simpson优势度指数随郁闭度增大而减小,Shannon-Wiener多样性指数、Pielou均匀性指数则显著增大,Margalef丰富度指数先显著增高(0.5—0.6)后趋于平稳(0.6—0.9);大型土壤动物Jaccard相似性指数低于中小型。5)CCA分析表明,不同类群土壤动物对环境因子响应不同,郁闭度、含水量、有机质及全P对土壤动物类群影响显著。研究表明,0.7郁闭度马尾松人工林下土壤动物优势度、丰富度适中,林下植被恢复情况较好,该郁闭度可能更有利于土壤动物多样性和群落稳定性。     

The switch of the binding behaviours between Xe and π system induced by the change of oxidation state of Cu ion

Abstract

The interaction between a xenon atom and aromatic π electron system is generally of van der Waals force with a specifically weak strength. In this work, we suggest the introduction of Cu ion will highly affect the binding behaviour between the xenon and π systems. Once Cu²⁺ ion locates above the benzene ring, the binding is surprisingly strengthened to 11.98 kcal mol^?1 at CCSD(T)/CBS level, which is significantly stronger than average strength of the H-bonds in Watson-Crick guanine-cytosine base pair. If the Cu²⁺ is reduced to Cu⁺, the interaction of interest returns to the weak van der Waals interaction again. This phenomenon indicates the oxidation state shift of Cu ion could regulate the binding strength of Xe with π systems, which would be important for their potential biological functions. This study may provide a plausible understanding of the recent experimental observations of xenon anaesthesia.     

Chromatography of Biological Materials on Polyethylene Glycol Treated Controlled-Pore Glass

Controlled-pore glass (CPG) columns will withstand high pressures without damage or loss of flow rates and their pore size and bed dimensions are independent of the eluent. Utilization of untreated CPG has been previously limited for permeation chromatography of various biological materials due to surface adsorption. Treatment of CPG with 1% polyethylene glycol (20, 000) solution prior to column preparation minimizes the adsorption phenomenon. High speed chromatography of proteins and enzymes was performed on treated CPG of various pore diameters. With some exceptions the proteins and enzymes eluted with greater than 92% recovery of the starting material and/or biological activity. Surface-treated CPG represents a useful tool for the characterization of many biological materials.     

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.     

Smart Monte Carlo Algorithm for the Adsorption of Molecules at a Surface

Abstract

A modified grand canonical ensemble Monte Carlo (GCMC) technique has been developed to simulate adsorption isotherms for molecules on or near a surface. The speed and accuracy of the simulation is increased by using a non-uniform distribution function, related to the force field exerted by the surface and the current configuration, to generate coordinates for the creation of new particles in the simulation. With this method, isotherms are generated more efficiently than with current techniques in which the creation step relies on a uniform distribution to generate the coordinates of a new molecule. This is shown by comparing the calculation of an isotherm for a simple molecule adsorbed on a graphite substrate from a traditional GCMC simulation with that calculated using this new technique.     

Semi-analytical mean-field model for predicting breathing in metal–organic frameworks

A new semi-analytical mean-field model is proposed to rationalise breathing of MIL-53 type materials. The model is applied on two case studies, the guest-induced breathing of MIL-53(Cr) with CO₂ and CH₄, and the phase transformations for MIL-53(Al) upon xenon adsorption. Experimentally, MIL-53(Cr) breathes upon CO₂ adsorption, which was not observed for CH₄. This result could be ascribed to the stronger interaction of carbon dioxide with the host matrix. For MIL-53(Al) a phase transition from the large pore phase could be enforced to an intermediate phase with volumes of about 1160–1300 Å³, which corresponds well to the phase observed experimentally upon xenon adsorption. Our thermodynamic model correlates nicely with the adsorption pressure model proposed by Coudert et al. Furthermore the model can predict breathing behaviour of other flexible materials, if the user can determine the free energy of the empty host, the interaction energy between a guest molecule and the host matrix and the pore volume accessible to the guest molecules. This will allow to generate the osmotic potential from which the equilibria can be deduced and the anticipated experimentally observed phase may be predicted.     

Experimental and theoretical study of the near IR emission of xenon excited by a fast electron beam

Emission of xenon excited by a 120-keV electron beam at gas pressures of 100, 200, 500, and 760 Torr nm was studied experimentally and theoretically. More than 30 spectral lines were identified in the wavelength range of 750–1000 nm. A self-consistent kinetic model is developed to calculate the emission intensity of xenon atoms in the near IR range. The model includes balance equations for the number densities of electrons, ions and excimer molecules; equations for the populations of electron levels; and the Boltzmann equation for the low-energy part of the electron energy distribution function with a source of slow electrons. Excitation and ionization rates of xenon by the beam electrons and the energy spectrum of slow electrons are calculated by the Monte Carlo method. It is shown that, under these conditions, the main mechanism of xenon atom excitation is dissociative recombination of Xe₃ ⁺ ions.     

Modelling Gas Adsorption in Slit-Pores Using Monte Carlo Simulation

Abstract

We discuss the use of Monte Carlo simulation to model the equilibrium adsorption of gases in slit pores. Databases of adsorption isotherms have been calculated for nitrogen, carbon-monoxide, methane and carbon-dioxide for a range of pressures, pore widths and temperatures. We discuss the implications of these results for materials characterisation procedures based on gas adsorption data.     

Opening of Bottleneck Pores for the Improvement of Nitrogen Doped Carbon Electrocatalysts

A facile synthesis strategy to control the porosity of ionothermal nitrogen doped carbons is demonstrated. Adenine is used as cheap and biomass based precursor and a mixture of NaCl/ZnCl₂ as combined solvent‐porogen. Variation of the ratio between the two salt influences the pore structure over a wide range. The eutectic mixture leads to micro‐ and mesoporous material with high total pore volume (TPV) of 3.0 cm³ g^?1 and very high surface area of 2900 m² g^?1 essentially rendering the product an “all‐surface‐area” nitrogen doped carbon. Increasing NaCl contents cause a continuous increase of the mesopore size and the formation of additional macropores resulting in a very high maximal TPV of 5.2 cm³ g^?1, showing 2540 m² g^?1 specific surface area using 60 mol% NaCl. Interestingly, the electrocatalytic activity of the samples toward oxygen reduction is strongly affected by the detailed pore structure. The different—however, chemically equivalent—catalysts vary up to 70 mV in their half wave potentials (E _1/2).The sample with optimized pore system shows a high selectivity toward the favored four electron process and an outstanding E _1/2 of ≈880 mV versus reversible hydrogen electrode (RHE), which is one of the best values reported for nitrogen doped carbons so far.     

Solubility of cadmium and cobalt in a post-oxic or sub-oxic sediment suspension

A sediment sample with high organic matter and trace metal content was suspended in synthetic river water for four weeks under an inert gas atmosphere. Subsequently, the anaerobic suspension was reoxidized by bubbling air through it. The concentrations of dissolved oxygen, sulfide, ferrous iron, manganese, cadmium, cobalt and the pH-value were measured at close time intervals during the anaerobic incubation. The anaerobic suspension was a post-oxic or sub-oxic environment with oxygen and total sulfide concentrations less than 1 µmole 1^–1. Concentrations of dissolved ferrous iron and manganese were 50–150 µmole 1^–1 and 5–30 µmole 1^–1, respectively. The total sulfide concentration was measured using a sensitive voltammetric technique, with a detection limit of 1 nmole 1^–1. A sequential extraction procedure was applied to two sediment samples taken at the end of the anaerobic incubation and after one week of reoxidation. The extractions indicated that cadmium was bound in sulfide minerals under post-oxic conditions. Thermodynamic equilibrium calculations revealed that the concentrations of dissolved cobalt in the post-oxic suspension were limited by the precipitation of cobalt sulfide minerals.     

The treatment of chromium tanning wastewater using natural marl

Abstract

The technical feasibility of removing chromium(III) from contaminated tannery wastewater by adsorption on naturally available marl, referred to as HEWAR in Palestine, has been investigated by batch experiments. The effects of various parameters on the percentage relative adsorption concentration were carried out by UV-vis spectrophotometry. Percentage relative concentration curves showed an increase with time until approaching a plateau (adsorption capacity). The adsorption capacity increased with the marl-to-liquid ratio, providing complete removal of chromium above 0.003g mL^?1 accomplished within 7 hours, while the marl particles were kept in suspension by stirring. The adsorption capacity increased with pH above 5.0, and decreased below this value. One sample of marl can be used for adsorbing chromium from various batches of wastewater until the adsorption sites are full.     

IFS-Simulation of Transport Phenomena on Complex Fractal Media

Abstract

We present in this work an off-lattice method of simulating transport phenomena on fractal structures based on the properties of Iterated Function Systems. The main advantages of this method, compared to classical lattice simulations are the facility of simulating transport processes on a great variety of fractal structures, the possibility of an accurate control of the lengthscale-approximation and the capability of simulating easily complex field-effects on the motion of diffusive particles. Some computer simulation data on fractal structures obtained with this method are presented.     

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.     

Vertical Ascending Electrophoresis of Cells with a Minimal Stabilizing Medium

Abstract

Vertical fractionation of a mixture of fixed horse and human red blood cells layered over a stabilizing support medium was done to give a valid comparison with proposed space experiments. In particular, the effects of sample thickness and concentration on zone migration rate were investigated. Electrophoretic mobilities of horse and human cells calculated from zone migration rates were compatible with those obtained by micro electrophoresis. Complete cell separation was observed when low power and effective cooling were employed.     

Neuropeptide cycloprolylglycine is an endogenous positive modulator of AMPA receptors

We have previously shown that neuropeptide cycloprolylglycine (CPG) increases the content of brain-derived neurotrophic factor (BDNF) in the culture of neuronal cells under normal conditions and in pathology. This is the first study to show that CPG at a physiological concentration of 10^–6 M significantly enhances the transmembrane AMPA currents in rat cerebellar Purkinje cells. Thus, CPG is a positive endogenous modulator of AMPA receptors. It was assumed that the neuropsychotropic effects of CPG are implemented as a result of BDNF accumulation after the activation of AMPA receptors by this neuropeptide.     

Synthesize of β‐cyclodextrin functionalized dendritic fibrous nanosilica and its application for the removal of organic dye (malachite green)

Dye removal from industrial waste water has become an important issue. The highvisibility, undesirability and recalcitrance are the significant environmental problemfor the dyes. In the present work,β‐cyclodextrin functionalized KCC‐1 (KCC‐1‐NH‐β‐CD)was synthesized and utilized to the removal of hazardous malachite green. In order to study the morphology of the synthesized nano adsorbent, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were obtained from the surface of the sample. Additionally, the functionalization of KCC‐1 with β‐cyclodextrin was confirmed with Furrier Transform Infrared spectroscopy (FTIR). The textural property of KCC‐1 was verified using nitrogen adsorption/ desorption analysis (BET equation). UV‐Vis spectroscopy utilized for the investigation of malachite green by KCC‐1‐NH‐β‐CD. Specific surface area of the adsorbent was calculated to be 140 m²/g and it can be stated that the synthesized nano adsorbent has high removal efficiency. It should be noted that the adsorption capacity of the employed nano adsorbent was more than 95%, which could be attributed to high porosity of β‐cyclodextrin functionalized KCC‐1.     

Modeling of adsorption in nanopores

Adsorption in nonporous materials has been studied using Grand Canonical Monte Carlo simulations. We discuss three types of materials: (a) a model of cylindrical pores with smooth walls, representing MCM-41 like materials, (b) a model of cylindrical pores with regular structured walls (model of carbon nanotubes) and (c) a material with crystalline wall structure (zeolites). Typical problems related to the stability of adsorbed layers have been analyzed. We have shown that the mechanism of adsorption is strongly dependent on the structure of the pore walls. In the case of amorphous walls it may lead to metastable configurations. In nanotubes, the ordered corrugation structure of walls determines the low temperature structure of the adsorbed system. In 3D ordered porous system, such as zeolites, the mechanism of adsorption is mostly determined by characteristic sites of adsorption.Figure Adsorbed atoms and energy fluctuations at the pressure of the first layer formation of krypton atoms: (a) instantaneous numbers of adsorbed atoms (per nm² of the pore wall) as a function of the time of simulation (Monte Carlo steps) observed in a relatively long run, (b) the bimodal distribution of the energy fluctuations is a consequence of the behavior of the systems as shown in (a).