Repositório Institucional da Universidade de Aveiro
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Study of strontium doping on the structural and magnetic properties of YMnO3ceramics
No full textStructural and Magnetic properties are investigated for the Sr doped YMnO3samples with different composition synthesized by a solid state reaction method. Sr doped YMnO3is the most distorted perovskite of the RMnO3 series (R=rare earths); the observed sinusoidal magnetic structure is in contrast with those exhibited by the less-distorted members, which are commensurate-type antiferromagnetic structures. A typical anti ferromagnetic (AFM) to paramagnetic (PM) phase transition is observed for the sample with concentration x = 0.12 and the Néel temperature (TN) is about 160 K. With decreasing temperature, the sample with x = 0.12 exhibit a magnetic transition from a paramagnetic (PM) to a ferromagnetic (FM) state. © 2013 VBRI press
Influence of the Crystal Structure on the Luminescence Properties of Mixed Eu,La-(1,10-Phenanthroline) Complexes
No full textA series of europium Eu- and La-doped complexes with two different crystal structures, namely, EuxLa1-x(phen)(2)(NO3)(3) [phen = 1,10-phenanthroline, x = 1.0 (1), 0.88 (2), 0.73 (3), and 0.52 (4)] and {[EuxLa1-x(phen)(2)(H2O)(2)(NO3)(2)](NO3)2(phen)(H2O)} [x = 0.28 (5) and 0.0 (6)], were synthesized in aqueous solution by using different Eu/La ratios.The complexes were characterized by single-crystal and powder X-ray diffraction, Fourier transform infrared spectroscopy, elemental analysis, thermogravimetry, and scanning electron microscopy. Compounds 1-4 and 5, 6 were obtained for Eu/La molar ratios higher than 5:5 and smaller than 3:7, respectively. The photoluminescence properties of these compounds depend on the crystal structure, which is determined by the Eu/La reactant ratio. Complexes 1-4 exhibit a quantum efficiency ( = 55%) and quantum yield (q = 31%) higher than 5 ( = 23% and q = 6%). The photophysical properties assessed by using the LUMPAC software are in good accord with the experimental values, particularly, the calculated (26%) and measured emission quantum yields (31%; excitation at the ligands at 255 nm)
Cs+ removal and optical detection by microporous lanthanide silicate Eu-AV-20 in a fixed-bed column
No full textMicroporous silicate Eu-AV-20 has recently proved to be a promising ion exchange material for cesium removal from aqueous solutions, and its potential for Cs+ photoluminescence sensing was additionally demonstrated. In this work, Cs+ removal was performed in a fixed-bed column, and the influence of linear velocity and mass of ion exchanger on the breakthrough curves was analysed. The experimental data were modelled on the basis of Nernst-Planck (NP) equations and with four well-known analytic models. The analytic expressions provided low errors (root mean square deviation, RMSD, between 3.20% and 6.47%); the 2-parameter NP-based model fitted the data quite well (RMSD = 6.66% for correlation and 6.54% for prediction), yielding crucial information on both the transport mechanism within the Eu-AV-20 particles, and the intrinsic dynamic behaviour of the fixed-bed ion exchange column. Taking into account that Eu-AV-20 samples loaded with different amounts of Cs+ exhibited distinct photoluminescence spectra, our results reinforce the potential of AV-20 materials for Cs+ sensing, which raises the possibility of online monitoring the ion exchange in a fixed-bed column using an optical fibre and a spectrometer. (C) 2015 Elsevier B.V. All rights reserved
Structural Characterization of Lignin from Grape Stalks (Vitis vinifera L.)
No full textThe chemical structure of lignin from grape stalks, an abundant waste of winemaking, has been studied. The dioxane lignin was isolated from extractive- and protein-free grape stalks (Vitis vinifera L.) by modified acidolytic procedure and submitted to a structural analysis by wet chemistry (nitrobenzene and permanganate oxidation (PO)) and spectroscopic techniques. The results obtained suggest that grape stalk lignin is an HGS type with molar proportions of p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units of 3:71:26. Structural analysis by H-1 and C-13 NMR spectroscopy and PO indicates the predominance of beta-O-4' structures (39% mol) in grape stalk lignin together with moderate amounts of beta-5', beta-beta, beta-1', 5-5', and 4-O-5' structures. NMR studies also revealed that grape lignin should be structurally associated with tannins. The condensation degree of grape stalks lignin is higher than that of conventional wood lignins and lignins from other agricultural residues
H-1 NMR studies of water- and alkaline-soluble organic matter from fine urban atmospheric aerosols
No full textSolution-state H-1 NMR spectroscopy was applied to characterize the structural features of water- and alkaline-soluble organic matter (WSOM and ASOM, respectively) sequentially extracted from fine urban atmospheric aerosols collected over different seasons. The objectives of this study were twofold: (i) to compare the functional characteristics of fine aerosol WSOM and the yet unknown ASOM, and (ii) to assess the sources of WSOM and ASOM at an urban location. The H-1 NMR analysis confirmed that the WSOM and ASOM fractions hold similar 1H functional groups; however, they differ in terms of their relative distribution throughout the seasons. The winter and autumn samples have a less aliphatic and oxidized character and higher contributions from aromatic groups than those of warmer conditions. The urban aerosol WSOM and ASOM fractions do not fit the established H-1 NMR source apportionment fingerprints of marine, secondary, and biomass burning organic aerosols (OAs), exhibiting a smaller relative contribution of carbon-oxygen double bonds, which is indicative of a less oxidized character. These results confirm that the boundaries of the pre-established source categories, especially for secondary and biomass burning OAs, might be different at urban locations, suggesting the need to improve the H-1 NMR source apportionment model for including a new fingerprint for urban OAs. (C) 2015 Elsevier Ltd. All rights reserved
Quantification of 3-deoxyglucosone (3DG) as an aging marker in natural and forced aged wines
No full textThe Maillard reaction product 3-deoxyglucosone (3DG) was quantified in wines, by high-performance liquid chromatography-mass spectrometry analysis after derivatization with ortho-phenylenediamine. Both sweet red Port wines and dry white wines were analysed during natural and forced aging. In natural aging, and for dry white wines, 3DG is negatively correlated to age (r=0.939), while for sweet red Port wines, 3DG is positively correlated to age (r=0.782). The same tendency was observed during a wine forced aging protocol. For a dry white wine, with higher levels of alpha-amino acids, 3DG is consumed (k(consumption) 0.077-0.098 day(-1)) along the time protocol, while for a sweet red Port wine, with lower levels of alpha-amino acids, an accumulation of 3DG is observed with time (k(formation) 0.041-0.060 day(-1)). These results suggest that 3DG content can be used as an aging marker, as it has discriminated dry white and sweet red Port wines from different ages and cultivars. Analysis of wine-model solutions allowed verifying that the fructose content has a higher effect on 3DG formation than glucose, as well as that an increase on amino acids content does not lead to an increase of 3DG yields. (C) 2016 Elsevier Inc. All rights reserved
Fractionation of phenolic compounds from lignin depolymerisation using polymeric aqueous biphasic systems with ionic surfactants as electrolytes
No full textLignin-based materials and chemicals represent a large class of potential added-value compounds for biorefineries; however, the difficulty in fractionating heterogenous phenolic compounds that result from lignin depolymerisation still remains a challenge. Herein, a set of polymer-based aqueous biphasic systems (ABS) comprising sodium polyacrylate (NaPA 8000) and polyethylene glycol (PEG 8000), in which ionic surfactants and ionic liquids are used as electrolytes at concentrations below 1 wt%, is investigated for this purpose. Binodal curves were determined for three electrolyte concentrations, namely, 0.01, 0.1 and 1.0 wt%, and an intermediate electrolyte concentration was selected to investigate the partition of five phenolic compounds, caffeic acid, vanillic acid, gallic acid, vanillin and syringaldehyde, on these systems. The results show that the ABS allow the manipulation of the partition of the five phenolic compounds, which culminates with their selective isolation, with vanillin selectivities of S-VN/VA = 23.75, S-VN/GA = 12.52 and S-VN/CA = 8.47. The electrolyte nature seems to play an important role in the fractionation of the phenolic compounds, due to the difference in their hydrophobic nature and speciation. This study demonstrates the ability of polymeric-based ABS, using ionic surfactants or ionic liquids as electrolytes, to efficiently separate and isolate five structurally similar phenolic compounds using a low-cost and very eco-friendly integrated process
Long range energy transfer in graphene hybrid structures
No full textIn this work we quantify the distance dependence for the extraction of energy from excited chromophores by a single layer graphene flake over a large separation range. To this end hybrid structures were prepared, consisting of a thin (2 nm) layer of a polymer matrix doped with a well chosen strongly fluorescent organic molecule, followed by an un-doped spacer layer of well-defined thicknesses made of the same polymer material and an underlying single layer of pristine, undoped graphene. The coupling strength is assessed through the variation of the fluorescence decay kinetics as a function of distance between the graphene and the excited chromophore molecules. Non-radiative energy transfer to the graphene was observed at distances of up to 60 nm; a range much greater than typical energy transfer distances observed in molecular systems
Evidence for the Interactions Occurring Between Ionic Liquids and Tetraethylene Glycol in Binary Mixtures and Aqueous Biphasic Systems
No full textThe well-recognized advantageous properties of poly(ethylene glycol)s (PEGs) and ionic liquids (ILs) in the context of an increasing demand for safe and efficient biotechnological processes has led to a growing interest in the study of their combinations for a wide range of procedures within the framework of green chemistry. Recently, one of the most promising and attractive applications has been the novel IL/polymer-based aqueous biphasic systems (ABS) for the extraction and purification of biomolecules. There still lacks, however, a comprehensive picture of the molecular phenomena that control the phase behavior of these systems. In order to further delve into the interactions that govern the mutual solubilities between ILs and PEGs and the formation of PEG/IL-based ABS, H-1 NMR spectroscopy in combination with classical molecular dynamics (MD) simulations performed for binary mixtures of tetraethylene glycol (TEG) and 1-alkyl-3-methylimidazolium-chloride-based ILs and for the corresponding ternary TEG/IL/water solutions, at T = 298.15 K, were employed in this work. The results of the simulations show that the mutual solubilities of the ILs and TEG are mainly governed by the hydrogen bonds established between the chloride anion and the -OH group of the polymer in the binary systems. Additionally, the formation of IL/PEG-based ABS is shown to be controlled by a competition between water and chloride for the interactions with the hydroxyl group of TEG
Ionic liquids for thiols desulfurization: Experimental liquid-liquid equilibrium and COSMO-RS description
No full textAiming at the replacement of the present inefficient and expensive desulfurization processes, ionic liquids have been considered as potential solvents to be used in extraction procedures. In this context, this work provides an experimental evaluation on the feasibility of ionic liquids for the selective extraction of a less studied class of aliphatic sulfur compounds - thiols. A mixture composed of n-dodecane and 1-hexanethiol was used, as a feed model of kerosene in \"jet-fuel'', and the tie-lines of the corresponding ternary systems were experimentally determined at 298.2 K and 313.2 K for imidazolium-and pyridinium-based ionic liquids. Using the experimental data, the selectivity and distribution ratios of 1-hexanethiol were also determined. Despite the small distribution ratios, these systems display a high selectivity meaning that the co-extraction of other fuel compounds can be controlled by the ionic liquids nature and/or chemical structural characteristics. The COnductor-like Screening MOdel for Real Solvents (COSMO-RS) was used to predict the liquid-liquid equilibrium of the investigated systems. A good agreement between the experimental data and the COSMO-RS results was observed. Therefore, the extraction behavior with other ionic liquids not experimentally addressed was also predicted by COSMO-RS for the identification of the best potential candidates. The ionic liquids identified are constituted by a short alkyl side chain imidazolium, pyridinium and pyrrolidium cations, combined with the anions tosylate, diethylphosphate, ethylsulfate and triflate. (C) 2014 Elsevier Ltd. All rights reserved