Molecular dynamics (MD) simulations and Brillouin light scattering (BLS) spectroscopy experiments have been carried to study the structure of sodium silicate glasses (SiO2)(100–X)(Na2O) X , where X ...ranges from 0 to 45 at room temperature. The MD-obtained glass structures have been subjected to energy minimization at zero temperature to extract the elastic constants also obtained by BLS spectroscopy. The structures obtained are in good agreement with the structural experimental data realized by different techniques. The simulations show that the values of the elastic constants as a function of X (i.e., Na2O mol %) agree well with those measured by BLS spectroscopy. The variations of elastic constants C 11 and C 44 as a function of Na2O mol % are discussed and correlated to structural results and potential energies of oxygen atoms.
In this study, we used ReaxFF reactive force field molecular dynamics (MD) simulations to investigate the dynamics associated with reactivity at the sodium aluminosilicate (NAS) glass–water ...interface. By combining van Hove correlation functions and visual analysis of individual trajectories, we found that when a Na+ ion leaves its initial position at the glass surface under the effect of water, it can be replaced either by a H+ ion or by another diffused Na+, resulting in a (Na+ ⇌ H+) ion exchange or (Na+ ⇌ Na+) ion exchange, respectively. These rearrangements at the NAS glass–water interface take place through many events such as the formation of oxygen-bearing functional groups (silanol Si(OH), germinal silanol Si(OH)2, and Al(OH)) and the agglomeration of Na+ ions on the glass surface. The high affinity between water and Na+ ions leads to two events, namely, the penetration of the water molecule into the first glass layer and the release of Na+ ions into the bulk water. The release of sodium ions into bulk water takes place via several successive steps: (1) leaving their original position to an area more exposed to bulk water, (2) diffusion to the surface of the glass, (3) leaving the surface toward the solution, and (4) coming back to the glass surface. Statistical analysis shows that oxygen-bearing functional group formation occurs both by protonation of the nonbridging oxygen (NBO) site and by filling defects such as SiIII, AlIII, and AlIV by OH– resulting from the dissociation of water molecules, while in the bulk, they could be produced by the proton jumps between two adjacent NBO sites and dissociation of the water diffused in deep layers of glass. Germinal silanol groups (Si(OH)2) are present in small amounts in the first layer of the glass after the protonation of two NBOs from a single SiIV. This process is accompanied by the conversion of AlIII and AlIV units into AlV units and SiIII units into SiIV units. By comparing the networks of Si and Al, we found that Al is more impacted by the occurrence of water molecules than Si, which is explained by an increase in the diffusion coefficient of Al compared with Si when these units become protonated (Si(OH) and Al(OH)).
The intrinsic Brønsted acid strength in solid acids relates to the energy required to separate a proton from a conjugate base, for example a negatively charged zeolite framework. The reliable ...characterization of zeolites’ intrinsic acidity is fundamental to the understanding of acid catalysis and setting in relation solid Brønsted acids with their activity and selectivity. Here, we report an infrared spectroscopic study with partial isotopic deuterium exchange of a series of 15 different acidic aluminosilicate materials, including ZSM-5 zeolites with very few defects. Varying Temperature Infrared spectroscopy (VTIR) permitted estimating activation energies for proton diffusion. Two different proton transfer mechanisms have been distinguished for two different temperature ranges. Si-rich zeolites appeared to be promising proton-transfer materials (E act. < 40 kJ mol–1) at temperatures above 150 °C (423 K). Further, a linear bathochromic shift of the Si–(OD)–Al stretching vibration as a function of temperature was observed. It can be assumed that this red-shift is related to the intrinsic O–(H/D) bond strength. This observation allowed the extrapolation and estimation of precise v(O–D)@0 K values, which could be attributed to distinct crystallographic locations through Density Functional Theory (DFT) calculations. The developed method was used to reliably determine the likelihood of the position of a proton in ZSM-5 zeolites under catalytically relevant conditions (T > 423 K), which has so far never been achieved by any other technique.
Theoretical and experimental assessments for the selective removal of phenol from synthetic 2nd generation biofuels. Investigation of the different structural interaction modes, their interaction ...energies, and vibrational frequencies.
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•Biofuels purification using zeolites was followed by experimental and theoretical approaches.•H+ and Na+ exchanged zeolites were compared for removing phenol from toluene.•DFT calculation combined with IR spectroscopy revealed the adsorption modes.•Multi-component breakthrough was conducted to investigate the adsorbents selectivity.•Astonishing performance of HY zeolite to remove phenol from a synthetic biofuel was shown.
The purification of second-generation biofuels is becoming an urgent issue due to the toxicity of the combustion products of residual phenol in these biofuels. The use of solid sorbents such as zeolites appears as a promising solution for ensuring the selective sorption of phenol towards aromatics (the main components of biofuel). In the present work, we have adopted a bottom-up approach for removing phenol from a synthetic biofuel feed containing isooctane, phenol (1 wt.%), n-nonane (1 wt.%) and toluene (40 wt.%), using faujasite-type Y zeolites with Si/Al ratio = 2.5. The astonishing performance of HY zeolite to treat the synthetic biofuel has been highlighted by assessing the interaction modes of the molecules involved over the structural sites via the combination of theoretical molecular modeling and experimental adsorption experiments.
In this study, density functional theory (DFT) calculations have been performed to investigate the adsorption mechanisms of toluene and water onto various cationic forms of Y zeolite (LiY, NaY, KY, ...CsY, CuY and AgY). Our computational investigation revealed that toluene is mainly adsorbed via π-interactions on alkalis exchanged Y zeolites, where the adsorbed toluene moiety interacts with a single cation for all cases with the exception of CsY, where two cations can simultaneously contribute to the adsorption of the toluene, hence leading to the highest interaction observed among the series. Furthermore, we find that the interaction energies of toluene increase while moving down in the alkaline series where interaction energies are 87.8, 105.5, 97.8, and 114.4 kJ/mol for LiY, NaY, KY and CsY, respectively. For zeolites based on transition metals (CuY and AgY), our calculations reveal a different adsorption mode where only one cation interacts with toluene through two carbon atoms of the aromatic ring with interaction energies of 147.0 and 131.5 kJ/mol for CuY and AgY, respectively. More importantly, we show that water presents no inhibitory effect on the adsorption of toluene, where interaction energies of this latter were 10 kJ/mol (LiY) to 47 kJ/mol (CsY) higher than those of water. Our results point out that LiY would be less efficient for the toluene/water separation while CuY, AgY and CsY would be the ideal candidates for this application.
Graphene oxide (GO) represents a complex family of materials related to graphene: easy to produce in large quantities, easy to process, and convenient to use as a basis for further functionalization, ...with the potential for wide-ranging applications such as in nanocomposites, electronic inks, biosensors and more. Despite their importance, the key structural traits of GO, and the impact of these traits on properties, are still poorly understood due to the inherently berthollide character of GO which complicates the establishment of clear structure/property relationships. Widely accepted structural models of GO frequently neglect the presence of extended topological defects, structural changes to the graphene basal plane that are not removed by reduction methods. Here, a combination of experimental approaches and molecular simulations demonstrate that extended topological defects are a common feature across GO and that the presence of these defects strongly influences the properties of GO. We show that these extended topological defects are produced following even controlled 'gentle' functionalization by atomic oxygen and are comparable to those obtained by a conventional modified Hummers' method. The presence of the extended topological defects is shown to play an important role in the retention of oxygen functional groups after reduction. As an exemplar of their effect on the physical properties, we show that the GO sheets display a dramatic decrease in strength and stiffness relative to graphene and, due to the presence of extended structural defects, no improvement is seen in the mechanical properties after reduction. These findings indicate the importance of extended topological defects to the structure and properties of functionalized graphene, which merits their inclusion as a key trait in simple structural models of GO.
In this study, we utilized molecular dynamics simulations with the embedded-atom approach to explore how different cooling rates and concentrations of chromium (Cr) affect the vitrification of ...metallic glasses (MGs) composed of a binary alloy FexCr100-x. We assessed this impact by analyzing the glass transition temperature (Tg) and the activation enthalpy (Δh). The results reveal an interesting non-linear trend in the activation energy. Specifically, there is a sharp decrease observed at a chromium concentration of 50%, followed by a slight increase at the 75% concentration level. These kinetic observations are linked to microstructural data obtained through Voronoi polyhedral analysis (VPA). However, it's worth noting that the influence of varying cooling rates on the vitrification process appears to be relatively modest.
The paper elucidates the main driving mechanisms at play during the early stage of the Ti/CuO thermite reaction using reactive forcefields in the frame of molecular dynamics calculations. Results ...show that TiO preferentially forms in immediate contact to pure Ti at temperatures as low as 200 K rather than TiO
2
. Increasing the temperature to 700 K, the 2 nm TiO
2
in contact to Ti is found to be homogeneously depleted from half of its oxygen atoms. Also, the first signs of CuO decomposition are observed at 600 K, in correlation with the impoverishment in oxygen atom reaching the titanium oxide layer immediately in contact to CuO. Further quantification of the oxygen and titanium mass transport at temperatures above 700 K suggests that mostly oxygen atoms migrate from and across the titanium oxide interfacial layer to further react with the metallic titanium fuel reservoir. This scenario is opposed to the one of the Al/CuO system, for which the mass transport is dominated by the Al fuel diffusion across alumina. Further comparison of both thermites sheds light on the enhanced reactivity of the Ti-based thermite, for which CuO decomposition is promoted at lower temperature, and offers a novel understanding of thermite initiation at large.
The paper elucidates the main driving mechanisms at play during the early stage of the Ti/CuO thermite reaction using reactive forcefields in the frame of molecular dynamics calculations.
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