Theoretical Calculations on Zeolites: The Aluminium Substitution in Mordenite,Ferrierite and ZSM-5

Abstract

The experimental determination of Al siting in zeolites involves the use of multiple techniques. Great effort has been made on both, experimental and theoretical approaches. The present study presents a novel methodology for calculating Al/Si replacement energies. Simple semiempirical calculations were applied on Modenite, Ferrierite and ZSM-5 zeolites, resulting in good agreement with the experimetal evidences. We have found that the favored Al substitution sites are T3 and T4 in Mordenite, while T2 and T4 are in Ferrierite, and only the T9 site is favored in ZSM-5. The method presented is based on an average of partial Al/Si replacement energies, evaluated for all rings belonging to each T site, rather than in the calculation of a total replacement energy evaluated for only one representative aggregate.     

Adsorption of arsenic on pre-treated zeolite at different pH levels

Abstract

Natural zeolites are widely used to minimise the impact of some pollutants in the environment, such as the removal of metals in various water sources as well as effluents used in the mining and metallurgical industry and in the treatment of mining acid solutions. The most common varieties of zeolites are analcime, chabazite, clinoptilolite, heroinite, ferrierite, mordenite. Researchers have studied for decades their synthesis, structure and proprieties, and as a result a large number of patents have been obtained on the preparation of different types of zeolite in conjunction with metals or metal compounds as selective adsorbents for anions and cations of water solutions. For example, iron, zirconium and magnesium hydroxide show selectivity in the adsorption of arsenic among other industrial applications of great interest.

This study shows the importance of using natural pre-treated zeolites in the adsorption of arsenic in water due to the adsorption percentages that are higher than 90%, controlling the pH in the pre-treatment. Samples were treated at different pH levels, getting a better adjustment of isotherms of adsorption at pH = 9 with ZP-9.     

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).     

A versatile modelling approach to determine the hydrophobicity of peptides at the atomic level

This study describes a versatile computational method to determine the hydrophobicity of small peptides at the atomic level. Free energies of transfer for individual atoms in peptide structures were derived, utilising two specifically defined parameters: (i) the water-excluding distance to define the dynamic interface between a peptide solute and its surrounding solvent and (ii) the corresponding hydrophobicity index as a relative measure for water occlusion/repulsion. The method was tested on a range of small peptide models (Ac-X-NH₂, G-X-G, Ac-WL-X-LL and Ac-GG-X-GG-NH₂) and several derivatives of these structures, whereby X was any of the 20 most common amino acids that naturally occur in polypeptides or proteins. The advantage of this new method lies in its versatility, ease to implement and capability to provide information on the hydrophobicity characteristics at the atomic level. The approach also encapsulates the impact of factors that influence these properties, but which have hitherto been difficult to accurately quantify, e.g. steric hindrance or proximity effects due to nearby polarised atoms. The method is not conditional on the knowledge of hydrophobicity parameters from the literature and does not require a sophisticated computer software/hardware to enable the atomic solvent-accessible surface areas or other hydrophobicity parameters to be de novo obtained.     

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].     

A field experiment on the use of Pistacia lentiscus L. and Scrophularia canina L. subsp. bicolor (Sibth. et Sm.) Greuter for the phytoremediation of abandoned mining areas

Abstract

A two-year study has been conducted in an abandoned Pb/Zn mining site, with the aim of investigating the feasibility of phytoremediation using two native Mediterranean plants (Pistacia lentiscus and Scrophularia bicolor) and of assessing the performance of amendments able to reduce the toxic effects of heavy metals. The amendments used were compost, chemical fertilizer, and zeolites, used singly or in combination. Depending on the amendments applied, the two species showed different mortality rates in the different plots, but all produced an increase in P. lentiscus survival, while S. bicolor survival improved only when amended with zeolite or zeolite + fertilizer. Scrophularia bicolor proved to be a more efficient accumulator than P. lentiscus, especially for Pb uptake. Pistacia lentiscus accumulated metals mostly in the roots. The effect of amendments was to generally reduce the bioavailable metal fraction, especially lead, in the plots amended with compost. Pistacia lentiscus proved to be the most suitable species for phytostabilization and environmental restoration, both for its resistance to metals and high phytomass production. The experiments demonstrate that the use of compost not only encourages this kind of revegetation in degraded areas, but is also an economical option that uses a by-product of solid municipal waste treatment.     

Influence of force fields on the selective diffusion of para-xylene over ortho-xylene in 10-ring zeolites

A molecular dynamics study of diffusion of p-xylene and o-xylene has been performed over three different pure silica 10-ring zeolites, MFI, SFG and TUN. The shape selective properties of the frameworks of these three materials have been tested using four different types of force fields commonly used based on united atom, rigid-ion and core-shell approximations. The performance of each force field is analysed in order to find which force fields can give sufficiently accurate estimations that allow to select appropriate zeolites for selective separation of para/ortho xylene. This performance was found to depend on the quality of the structural properties of the zeolite, in particular the size and shape of the 10 rings which act as bottlenecks for the diffusion. The computational results allow us to define some optimum characteristics for the selective diffusion of p-xylene.     

Dissociation of Aun (n = 3, 4) Microclusters

Abstract

The dissociation of Au₃ and Au₄ microclusters has been investigated using a molecular-dynamics computer simulation. The potential energy function used in the calculations includes two- and three-body interactions, which are represented by Lennard-Jones and Axilrod-Teller potentials, respectively. It has been found that dissociation starts for both clusters after 3000K.     

Molecular dynamics (MD) simulations and binding free energy calculation studies between inhibitors and type II dehydroquinase (DHQ2)

Type II dehydroquinase (DHQ2) is the third enzyme of the shikimic acid pathway, and it has been the effective target for tuberculosis (TB). So far, developing multiple potent inhibitors of the DHQ2 of Mycobacterium tuberculosis (DHQ2-Mt) has been considered to be the new therapy to TB. Molecular dynamics simulations followed by molecular mechanics-generalised Born surface area were carried out to calculate the free binding energy and to determine the affinity ability of the four chosen inhibitor molecules, L1, L2, L3 and L4. Energy decomposition analyses show the electrostatic interaction and van der Waals interaction of the ligands to every residue of the DHQ2-Mt. The results suggest that some important residues have different interactions with the four ligands, such as Arg19 and Tyr24. These interactions may have an effect on the ligand binding affinity. The binding affinity of monosubstituted inhibitor is higher than that of disubstituted inhibitor, due to some important interactions with the DHQ2-Mt residues. These computational works will be significant to the theoretical research in the future.     

Molecular Simulation of Adsorption of Guest Molecules in Zeolitic Materials: A Comparative Study of Intermolecular Potentials

Abstract

Grand Canonical Monte Carlo simulation, together with an appropriate guest-host forcefield is shown to provide reasonably accurate predictions of adsorption properties of guest molecules in a variety of zeolitic materials. The use of a simple guest-host Kiselev-type potential permits the calculations to capture the essence of the behavior of simple guest-host systems such as rare gases or methane molecules in neutral AlPO₄-5. However, a full scale potential is needed in the more complex cases of large anisotropic molecules adsorbed in cationic zeolites (such as xylene isomers in faujasite). The guest-host potential model developed by Nicholson and coworkers is shown to allow an excellent transferability of the potential parameters from one guest/host system to another.     

Pin-photodiode array for the measurement of fan-beam energy and air kerma distributions of X-ray CT scanners

PurposePatient dose estimation in X-ray computed tomography (CT) is generally performed by Monte Carlo simulation of photon interactions within anthropomorphic or cylindrical phantoms. An accurate Monte Carlo simulation requires an understanding of the effects of the bow-tie filter equipped in a CT scanner, i.e. the change of X-ray energy and air kerma along the fan-beam arc of the CT scanner. To measure the effective energy and air kerma distributions, we devised a pin-photodiode array utilizing eight channels of X-ray sensors arranged at regular intervals along the fan-beam arc of the CT scanner.MethodsEach X-ray sensor consisted of two plate type of pin silicon photodiodes in tandem – front and rear photodiodes – and of a lead collimator, which only allowed X-rays to impinge vertically to the silicon surface of the photodiodes. The effective energy of the X-rays was calculated from the ratio of the output voltages of the photodiodes and the dose was calculated from the output voltage of the front photodiode using the energy and dose calibration curves respectively.ResultsThe pin-photodiode array allowed the calculation of X-ray effective energies and relative doses, at eight points simultaneously along the fan-beam arc of a CT scanner during a single rotation of the scanner.ConclusionsThe fan-beam energy and air kerma distributions of CT scanners can be effectively measured using this pin-photodiode array.     

定陶汉墓出土汉砖表面及其储存环境真菌病害的分析与防治

【背景】定陶王墓地M2汉墓出土了一批处于防护层的重要汉砖,目前部分汉砖表面出现了真菌病害问题。【目的】根据现场文物保护状况,选取了5块汉砖表面的微生物病害样本,检测并分析了汉砖表面的真菌病害,将分离得到的7株真菌进行抑菌试验。【方法】使用扫描电镜观察、高通量测序、纯培养和抑菌试验等方法对样本中的真菌进行分析。【结果】发现未经抑菌处理的汉砖表面真菌病害问题严重,经75%乙醇和0.5%硝酸咪康唑处理过的汉砖表面真菌病害问题减弱。汉砖上的主要真菌不同,汉砖9表面含量最多的真菌为Boeremia,汉砖13表面含量最多的为虫草科(Cordycipitaceae)。然而硝酸咪康唑和硼酸硼砂并无抑菌效果,抑菌产品K100(2-甲基-4-异噻唑啉-3-酮)具有较好的抑菌效果。【结论】本研究对分析和防治汉砖表面的真菌病害及对汉砖进行科技保护具有重要意义。     

基于近10年红球菌发展的文献计量分析

【背景】近些年,越来越多的研究集中在红球菌上,很少有研究人员对现有文献进行全面回顾。【目的】为了探究国内外红球菌领域的研究热点、前沿和未来发展趋势,以便为后续研究人员提供全面直观的参考。【方法】对近10年发表在Web of Science的红球菌领域论文进行统计分析和文献计量分析。通过VOSviewer文献可视化软件绘制作者标签视图和关键词共现网络图。【结果】全球有关红球菌领域的发文量总体呈逐年上升趋势,发表期刊多为微生物学领域的专科期刊,中国和美国的文章发表数和引用数远超其他国家,红球菌的研究内容也主要集中在生物催化、生物降解和非核糖体肽合成酶等方面。【结论】红球菌在世界范围内越来越受到重视,并逐渐成为研究热点,国家和研究机构应继续加强合作,推动红球菌领域的继续发展。     

Non-invasive characterization of coronary artery atherosclerotic plaque using dual energy CT: Explanation in ex-vivo samples

PurposeIn this study non-calcified plaque composition is evaluated by Dual Energy CT (DECT). Energy Dispersive X-ray Spectroscopy (EDS) has been used to study the Plaque composition. An attempt has been made to explain the DECT results with EDS analysis.MethodsThirty-two ex-vivo human cadaver coronary artery samples were scanned by DECT and data was evaluated to calculate their effective atomic number and electron density (Z_eff & ρ_e) by inversion method. Result of DECT was compared with pathology to assess their differentiating capability. The EDS study was used to explain DECT outcome.ResultsDECT study was able to differentiate vulnerable plaque from stable with 87% accuracy (area under the curve (AUC):0.85 [95% confidence interval {CI}:0.73–0.98}] and Kappa Coefficient (KC):0.75 with respect to pathology. EDS revealed significant compositional difference in vulnerable and stable plaque at p < .05. The weight percentage of higher atomic number elements like F, Na, Mg, S, Si, P, Cl, K and Ca was found to be slightly more in vulnerable plaques as compared to a stable plaque. EDS also revealed a significantly increased weight percentage of nitrogen in stable plaques.ConclusionsThe EDS results were able to explain the outcomes of DECT study. This study conclusively explains the physics of DECT as a tool to assess the nature of non-calcified plaques as vulnerable and stable. The method proposed in this study allows for differentiation between vulnerable and stable plaque using DECT.     

Immobilization of Nickel by Bacterial Isolates from the Indian Ridge System and the Chemical Nature of the Accumulated Metal

Bacterial isolates from Carlsberg-Ridge waters were tested for their ability to immobilize Ni. For this, test cultures were suspended for 60 d in seawater with and without added Ni at concentrations of 10 to 10,000 μ M. At an initial concentration of 100 μ M, isolates CR35 and CR48 caused a 89.8 and 6.95 μ M decrease in dissolved Ni at 28 2°C and a 14.75 and 6.38 μ M at 3 1°C. Analyses showed the contribution of hydroxyl, carbonyl, sulfide and phosphoryl groups in Ni binding. The study proves that Ni immobilization can occur even in the absence of Mn and suggests their plausible role in the ridge ecosystem.     

Exploring the nature of the H-bonds between the human class II MHC protein,HLA-DR1 (DRB*0101) and the influenza virus hemagglutinin peptide,HA306-318, using the quantum theory of atoms in molecules

The nature of the H-bonds between the human protein HLA-DR1 (DRB*0101) and the hemagglutinin peptide HA306-318 has been studied using the Quantum Theory of Atoms in Molecules for the first time. We have found four H-bond groups: one conventional CO··HN bond group and three nonconventional CO··HC, π··HC involving aromatic rings and HN··HC_aliphatic groups. The calculated electron density at the determined H-bond critical points suggests the follow protein pocket binding trend: P1 (2,311) >> P9 (1.109) > P4 (0.950) > P6 (0.553) > P7 (0.213) which agrees and reveal the nature of experimental findings, showing that P1 produces by a long way the strongest binding of the HLA-DR1 human protein molecule with the peptide backbone as consequence of the vast number of H-bonds in the P1 area and at the same time the largest specific binding of the peptide Tyr308 residue with aromatic residues located at the binding groove floor. The present results suggest the topological analysis of the electronic density as a valuable tool that allows a non-arbitrary partition of the pockets binding energy via the calculated electron density at the determined critical points.     

Diffusion of N2, O2, H2S and SO2 in MFI and 4A zeolites by molecular dynamics simulations

This paper presents diffusion data of N₂, O₂, H₂S and SO₂ in MFI and 4A zeolites obtained by molecular dynamics simulations, especially its dependence on temperature and loading. At high loadings and temperatures, the order of self-diffusivity of guests in two zeolites is O₂ > N₂ > H₂S>SO₂. The diffusion behaviour is different in different zeolites at lower loadings, reflecting different influences from straight channels (MFI) and α-cages (4A). Furthermore, with increasing loading, the self-diffusivity of guest molecules decreases in MFI but generally increases in 4A. The centre of mass (COM) probability densities and diffusion trajectories of guests give insight into molecular-level diffusion process. The simulation results reveal that with increasing loading, the diffusion mechanism would change from the inter-pore to intra-pore diffusion in MFI. However, in 4A, the intra-pore diffusion is predominant at low and high loadings, but inter-pore diffusion is more important at moderate loadings.     

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.     

Effects of partial sleep deprivation after prolonged exercise on metabolic responses and exercise performance on the following day

[Purpose]We determined the effect of partial sleep deprivation (PSD) after an exercise session on exercise performance on the following morning.[Methods]Eleven male athletes performed either a normal sleep trial (CON) or a PSD trial. On the first day (day 1), all subjects performed an exercise session consisting of 90 min of running (at 75% V˙O2max) followed by 100 drop jumps. Maximal strength (MVC) was evaluated before and after exercise. In the CON trial, the sleep duration was 23:00–7:00, while in the PSD trial, the sleep duration was shortened to 40% of the regular sleep duration. On the following morning (day 2), MVC, the metabolic responses during 20 min of running (at 75% V˙O2max), and time to exhaustion (TTE) at 85% V˙O2max were evaluated.[Results]On day 2, neither the MVC nor V˙O2 during 20 min of running differed significantly between the two trials. However, the respiratory exchange ratio was significantly lower in the PSD trial than in the CON trial (p = 0.01). Moreover, the TTE was significantly shorter in the PSD trial than in the CON trial (p = 0.01).[Conclusion]A single night of PSD after an exercise session significantly decreased endurance performance without significantly changing muscle strength or cardiopulmonary response.