•Thermal dehydration/decomposition of metakaolin-based geopolymer proceeds in multistep process.•The process comprises the desorption of water vapor and subsequent two-step reactions.•Desorption of ...water vapor separates into two steps at a high water vapor pressure.•The kinetics of the first reaction step is significantly influenced by the atmospheric water vapor.•The second reaction step is less sensitive to the atmospheric water vapor.
This study investigated the kinetics of the multistep thermal dehydration/decomposition of the metakaolin-based geopolymer paste. The component two reaction steps were characterized by the evolution of water vapor and the simultaneous evolution of water vapor and CO2, respectively. In a stream of dry N2, the kinetics of the first and second reaction steps were characterized by the apparent activation energy (Ea) values of 92 and 166 kJ mol−1, respectively. Both reaction steps exhibited a diffusion-controlled rate behavior. In a stream of wet N2, the mass loss curves systematically shifted to higher temperatures with an increase in the water vapor pressure (p(H2O)). The first reaction step was significantly influenced by p(H2O), and the apparent Ea increased to 175 kJ mol−1 at p(H2O) = 11.4 kPa. The second reaction step was less sensitive to the atmospheric water vapor, as characterized by its Ea of ∼165 kJ mol−1, irrespective of the p(H2O).
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Li1+xAlxTi2−x(PO4)3 (0.1 ≤ x ≤ 0.6) powders were prepared by a novel sol-gel method with high phase purity, densification activity and conductivity. Diffraction analyses showed that the solubility ...limit of Al3+ was reached at x = 0.5. The highest ionic conductivity was obtained for Li1.5Al0.5Ti1.5(PO4)3, which reached 1.0 × 10−3 S cm−1 at 25 °C when tested in ambient air. However, measurements in dry argon resulted in a conductivity of only 5 × 10−4 S cm−1 at 25 °C. Hence, the influence of moisture or water on microstructure and grain-boundary conductivity was investigated by impedance spectroscopy, scanning electron microscopy, Raman spectroscopy, X-ray and neutron diffraction. An ion exchange of Li+ by protons could not be unambiguously achieved by exposure of LATP powder in water. Neutron diffraction of humidified samples did not clearly indicate the presence of water or protons in the crystal structure of LATP, whereas μ-Raman measurements confirmed the presence of water/protons on the sample surface and in the bulk material. A higher signal of the vibrational modes of H2O was measured on grain boundaries than in the grain interior on the sample surface as well as on the fractured surface of sintered specimens. Therefore the higher apparent conductivity of LATP samples may predominantly result from adsorbed water from ambient air at grain boundaries. Hence, the conductivity tested in dry argon represents the correct conductivity of LATP samples. The highest density after sintering is not necessarily leading to high conductivity but rather the microstructure plays the dominant role.
•Preparation of highly dense Li1.5Al0.5Ti1.5(PO4)3•Influence of water on it•Characterization of protons with SEM, Raman and neutron diffraction
Aldol condensation of furfural and acetone (an important initial step to obtain diesel from biomass) was studied over MgAl mixed oxides. The influence of the utilization of microwaves and/or a ...surfactant (Pluronic 123) during the synthesis as well as the use of water (either pre-hydrating the solids before catalytic studies or in water/toluene mixtures as the reaction medium) is discussed. The combined use of Pluronic 123 and microwaves led to solids with bigger pore sizes, exhibiting lower basicity and higher acidity than the conventional synthetic method, thus resulting in an increase in the yield of the desired product of condensation, comprising two molecules of furfural and one of acetone (F2Ac). As for the influence of water, re-hydration of the mixed oxides was detrimental to activity, probably as a result of the partial blocking (solvation) of active sites. On the contrary, the increase in water percentage in the reaction medium resulted in higher conversions, though selectivity to F2Ac decreased. The weakening of the C=O bond of furfural in the presence of water as well as the higher solubility of the first condensation product (FAc) in toluene, as compared to water, could account for that. A 44.5% yield of F2Ac (66% conversion) after 16 h was obtained with the most active solid, which maintained the activity for three consecutive reactions.
Three different catalysts, namely Pt/γ-Al2O3, Au/α-Fe2O3 and CuO–CeO2 were prepared (by wet impregnation, coprecipitation and a sol–gel method, respectively) and their catalytic performance for the ...selective oxidation of carbon monoxide in the presence of excess hydrogen was evaluated and compared. The effects of the presence of CO2 and H2O in the reactant feed on the activity and selectivity of these catalysts, as well as their stability under realistic reaction conditions were also investigated.
Regardless of whether CO2 or both CO2 and H2O are present in the reactant feed, the Au/α-Fe2O3 catalyst is superior to the other two for the selective CO oxidation at relatively low reaction temperatures (<80–120°C, depending on contact time and feed composition employed), while at higher reaction temperatures, best results are obtained with the CuO–CeO2 catalyst, which proved to be more active and remarkably more selective than the Pt/γ-Al2O3 catalyst. The Au/α-Fe2O3 catalyst was the most sensitive, while the Pt/γ-Al2O3 the most resistant towards deactivation caused by the presence of CO2 and H2O in the feed. Finally, while the Au/α-Fe2O3 catalyst lost a considerable portion of its activity during the first 80h under reaction conditions, the CuO–CeO2 and Pt/γ-Al2O3 catalysts exhibited a stable catalytic performance, at least during the time period tested in this work (7–8 days).
In order to develop a method for assessing the service life, the values of kinematic viscosity at temperatures of 100 °C and minus 40 °C, acid number and amount of sediment insoluble in isooctane, ...critical load and the wear scar diameter, height and time of destruction of the foam column, as well as the tensile strength of IRP-1316, IRP-1287 and 51-1434 sealing rubbers of aviation oils were determined in the state of delivery and after oxidation at temperatures of 200, 225, and 240 °C. The influence of water in concentrations of 0.01, 0.1 and 1% on the performance properties of oils, was estimated. Based on the mathematical dependences obtained for the property values on the time and temperature of oxidation, the estimation of the operating time of the standard oil for aircraft gas turbine engines and helicopter gearboxes was carried out
Methyl lactate (MLA), a versatile biomass platform, was typically produced from the catalytic conversion of high-priced fructose. High fructose corn syrup (HFCS) is a mixture of glucose, fructose, ...water, etc., which is viewed as an economical substitute for fructose to produce MLA due to the much lower cost of separation and drying processes. However, the transformation of HFCS to MLA is still a challenge due to its complex components and the presence of water. In this work, the catalytic conversion of HFCS to MLA over CoO@silicalite-1 catalyst synthesized via a straightforward post citric acid treatment approach was reported. The maximum MLA yield reached 43.8% at 180 °C for 18 h after optimizing the reaction conditions and Co loading. Interestingly, adding extra 3% water could further increase the MLA yield, implying that our CoO@silicalite-1 catalyst is also capable for upgrading wet HFCS. As a result, the costly drying process of wet HFCS can be avoided. Moreover, the activity of CoO@silicalite-1 catalyst can be regenerated for at least four cycles via facile calcination in air. This study, therefore, will provide a new opportunity to not only solve the HFCS-overproduction issues but also produce value-added MLA.
Tilt fluctuations can potentially reflect the response of hydrosystems to important rainfall. In this context, long baseline tiltmeters have been installed in an underground tunnel penetrating the ...Fontaine de Vaucluse karst to study the medium deformation related to solicitations exerted by water infiltrating the hydrosystem. The instruments monitor the tilt as well as its spatial variation. Northward tilts reaching a 1 μrad amplitude are observed consecutively to rainfalls. The tilt amplitude is highly correlated with the Fontaine de Vaucluse outlet flow fluctuations. The measured tilt signal is also relatively homogeneous over a 150 m length. Different types of structure likely to produce such observations are tested in order to identify their location with respect to the tiltmeters, their dimension as well as the amount of water level variation in the structure. Following rainfalls, the infiltration of water modifies the pore pressure, inducing a medium deformation. The hypothesis of an homogeneous surface loading on the Vaucluse plateau is first refuted since the related tilt is much lower than the one measured. The water supplied by rainfalls has to accumulate in discontinuities in order to generate a higher tilt. So, the deformation related to a pressure exerted on a fracture filled by water is assessed. A first study reveals the interest of the tilt homogeneity information that constrains strongly the fracture properties. Thus, the fracture must be located at a distance more than a few hundreds metres from the tiltmeters in order to produce a tilt homogeneous in space. If the fracture is initially dry, it must also be filled on a height higher than 150 m consecutive to a rainfall in order to generate a tilt amplitude in the same magnitude as the one measured. Then, we explore the influence of water level variations on the tilt produced by a fracture located at the interface between the saturated and unsaturated zones, which are thereby permanently flooded. Since several parameters of that model satisfactorily explain the field observations, we discuss how simultaneous geodetical observations could provide complementary information that would further constrain the geometry of the structure at the origin of the medium deformation.
A complete understanding of the physics underlying the varied colors of firefly bioluminescence remains elusive because it is difficult to disentangle different enzyme-lumophore interactions. ...Experiments on isolated ions are useful to establish a proper reference when there are no microenvironmental perturbations. Here, we use action spectroscopy to compare the absorption by the firefly oxyluciferin lumophore isolated in vacuo and complexed with a single water molecule. While the process relevant to bioluminescence within the luciferase cavity is light emission, the absorption data presented here provide a unique insight into how the electronic states of oxyluciferin are altered by microenvironmental perturbations. For the bare ion we observe broad absorption with a maximum at 548 ± 10 nm, and addition of a water molecule is found to blue-shift the absorption by approximately 50 nm (0.23 eV). Test calculations at various levels of theory uniformly predict a blue-shift in absorption caused by a single water molecule, but are only qualitatively in agreement with experiment highlighting limitations in what can be expected from methods commonly used in studies on oxyluciferin. Combined molecular dynamics simulations and time-dependent density functional theory calculations closely reproduce the broad experimental peaks and also indicate that the preferred binding site for the water molecule is the phenolate oxygen of the anion. Predicting the effects of microenvironmental interactions on the electronic structure of the oxyluciferin anion with high accuracy is a nontrivial task for theory, and our experimental results therefore serve as important benchmarks for future calculations.
In the Volga River water, sulfate ions rank second among the main anions in terms of their contribution to water mineralization. To study the seasonal variability of sulfate concentration, an ...analysis of long-term data of hydrological and hydrochemical observations obtained on the River Volga (in the outlet section of the Kuibyshev reservoir) has been carried out. For the period of 2001-2018 the average annual water consumption in the dam section varied in the range of 6.2-9.0 thousand m3/s, the average annual concentration of sulfates was 55 mg/dm3, the highest - 64 mg/dm3, and the lowest - 45 mg/dm3. The content of sulfates in the reservoir is formed mainly under the influence of the Volga River water coming from the Cheboksary reservoir and the Kama River water coming from the Nizhnekamsk reservoir. The sulfate content is characterized by significant seasonal variability. During the winter low-water period, the highest concentrations of sulfates were observed with a maximum in April, before the beginning of the spring flood (67 mg/dm3). During the flood, the content of sulfates decreased, reaching the lowest values during the summer low-water period in August (44 mg/dm3), and then, from September, the concentration of sulfates gradually increased, reaching 57 mg/dm3 at the beginning of the winter low-water period. Over a long-term observation period, the amplitude of sulfate fluctuations was 34-87 mg/dm3, and its value depended on the water content of a particular year. In dry years, the concentration of sulfates in the water increased, and in dry years, it decreased. In the seasonal context, the main differences in sulfate concentrations in dry and high-water years occurred during the spring flood and winter low-water period, and during the summer - autumn low-water period, the differences became minimal.
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