Papers by Alexandre Leitão
Chemistry of Materials, 2021
In addition to a great swelling ability, layered silicates also allow the functionalization of th... more In addition to a great swelling ability, layered silicates also allow the functionalization of their interlayer region to form various robust green materials that are used as CO 2 adsorbents, drug carriers, or catalysts. Here, the unique magadiite structure, which has resisted elucidation despite many attempts and applications the material offers, is finally described. A materialspecific strategy allowed the use of 3D electron diffraction which led to the success of deciphering the atomic structure. In order to enable an ab initio structure solution of the electron beam sensitive material, a sodium-free dehydrated form of magadiite was synthetically isolated, and, from that, it was subsequently possible to derive a structure model for the sodium form of magadiite, later successfully refined against powder X-ray diffraction data. Furthermore, a geometry optimization and simulations of spectroscopic data with DFT methods confirm the obtained crystal structure of sodium magadiite. These results finally prompted a detailed description of the layers and of the chemically active interlayer region and provide a huge impact toward the design of new and more efficient materials based on functionalized magadiite and related structures.
Acta Crystallographica Section A Foundations and Advances, 2021
Revista Virtual de Química, Apr 14, 2014
The Pró-Defesa funding of projects was a joint initiative of CAPES and the Ministry of Defense to... more The Pró-Defesa funding of projects was a joint initiative of CAPES and the Ministry of Defense to fund research in defense. In this article we review the main scientific results obtained in the project "Formação de Pessoal Qualificado em Química Quântica Computacional para Atuação na Área de Defesa Química", funded by the first edition of Pró-Defesa in 2005, which involved a collaboration between the Departments of Chemistry of the Federal University of Juiz de Fora and of the Military Institute of Engineering. The main goal of this project was to study, using theoretical-computational methods, the potential of magnesium oxide and hydroxide surfaces to degrade organophosphorus (OP) substances used as warfare agents, also known as neurotoxics. These substances inhibit the acetylcholinesterase, enzyme of fundamental role in the central nervous system. The VX (O-ethyl-S-(2-diisopropylethylamino) ethyl methylphosphonothioate) and sarin (isopropyl methylphosphonofluoridate) agents are some of the main neurotoxic agents, thus the search for ways to degrade them is considerably important. In this paper, we present theoretical investigations of two chemical degradation processes of organophosphates: the hydrolysis of a VX-like OP compound (O,S-dimethyl methylphosphonothioate, DMPT) by the dissociative chemisorption on the MgO(001) surface and the degradation of sarin by a surface of magnesium hydroxide Mg(OH) 2 , known as brucite. These processes were studied by the combination of density functional theory (DFT) with periodic boundary conditions, an approach that is the state of the art of the theoretical methods for this type of problem. In the case of DMPT, we proposed a degradation mechanism that involves degradation reactions of DMPT and water molecules in the presence of two types of MgO(001) surface models: terrace and Al-doped. Conformations, free energy differences, transition states and reaction barriers were calculated. We showed that the MgO(001) surface acts as a possible catalyst for the degradation of VX, but with higher selectivity than the Al-doped surface when compared with sites without defects on the terrace. These results may have important applications as well as to serve as reference for further studies on the decomposition of VX. In the degradation process of sarin using brucite, we proposed four elementary reactions for the global mechanism. We analyzed the adsorption of sarin on the brucite layer through the fluorine and the phosphonyl oxygen atoms (P=O). The two transition states correspond to a hydroxyl anion motion toward the phosphorus atom and to a fluoride moving toward the hydroxyl vacant position; the activation barrier for the rate-limiting step, corresponds to adsorbed sarin attacked by the brucite hydroxyl. The products of the global reaction were an isopropyl methylphosphonate molecule and [Mg(OH) 2-x ]F x . The results of the deactivation process of sarin using a brucite surface show the potential of layered hydroxides to degrade OP compounds. Also the results of this research have a dual character because several pesticides are organophosphorous compounds. Furthermore, the theoretical approach employed can be used for various chemicals processes to get quite accurate results.
Journal of Pharmaceutical Sciences, 2016
Revista Virtual de Química, 2014
Physical Review B, 2011
The Gauge-Including Projector Augmented Wave (GIPAW) method, within the Density Functional Theory... more The Gauge-Including Projector Augmented Wave (GIPAW) method, within the Density Functional Theory (DFT) Generalized Gradient Approximation (GGA) framework, is applied to compute solid state NMR parameters for 27 Al in the α, θ, and κ aluminium oxide phases and their gibbsite and boehmite precursors. The results for well-established crystalline phases compare very well with available experimental data and provide confidence in the accuracy of the method. For γ-alumina, four structural models proposed in the literature are discussed in terms of their ability to reproduce the experimental spectra also reported in the literature. Among the considered models, the F d3m structure proposed by Paglia et al. [Phys. Rev. B 71, 224115 (2005)] shows the best agreement. We attempt to link the theoretical NMR parameters to the local geometry. Chemical shifts depend on coordination number but no further correlation is found with geometrical parameters. Instead our calculations reveal that, within a given coordination number, a linear correlation exists between chemical shifts and Born effective charges.
Journal of the Brazilian Chemical Society, 2021
Synthesis of glycerol carbonate over a 2D coordination polymer built with Nd<sup>3+</sup>ions and organic ligands
Dalton Transactions, 2018
In this work, a two-dimensional coordination polymer was synthesized and the structure was determ... more In this work, a two-dimensional coordination polymer was synthesized and the structure was determined by single-crystal X-ray diffraction. The crystal structure belongs to the space group Pna21 and was characterized by Raman and FT-infrared spectroscopy, powder X-ray diffraction and Brunauer-Emmett-Teller surface area analysis. Catalyst activities were evaluated through the synthesis of glycerol carbonate from glycerol and urea using a batch reactor. After the optimization of both reaction and reaction conditions, the activity results showed that the coordination polymer used as a heterogeneous catalyst has good values of conversions and selectivity for the manufacturing of glycerol carbonate in a fine-chemical process. The analysis of powder X-ray diffraction and spectroscopy for the coordination polymer employed, before and after the reaction, shows that some changes have taken place in the crystal structure during the process, in spite of a recovery at the end of the reaction. The advantages and limitations of the coordination polymer were discussed and compared with those of the previous heterogeneous catalysts in the literature.
Applied Clay Science, Nov 1, 2015
In this work, the influence of the zinc content and synthesis methodology on the physicochemical ... more In this work, the influence of the zinc content and synthesis methodology on the physicochemical properties of mixed oxide derivatives from the hydrotalcite precursor MgZnAl-HT were evaluated. Layered double hydroxides (LDH) were prepared by co-precipitation, ultrasound and urea hydrolysis methods followed by calcination at 500°C. The materials were studied by such techniques as inductively coupled plasma, X-ray powder diffraction, Raman spectroscopy and 27 Al solid-state NMR spectroscopy. The textural and morphological properties of mixed oxides were evaluated through the adsorption/desorption of N 2 and scanning electron microscopy (SEM). Different textural and morphological properties were observed for the samples.
Applied Clay Science, Jun 1, 2016
Ab initio calculations based on density-functional theory (DFT) were made to a large ensemble of ... more Ab initio calculations based on density-functional theory (DFT) were made to a large ensemble of representative models of dehydrated smectites with Na + as counterion. The structures were generated by different isomorphic substitutions of Al 3+ by Mg 2+ in octahedral sheet and Si 4+ by Al 3+ in tetrahedral sheet followed by full optimization of the geometries. Structures were chosen among the lowest total energies to calculate nuclear magnetic resonance (NMR) parameters of 1 H, 27 Al, 29 Si, 25 Mg and 23 Na by means of gauge including projector augmented wave (GIPAW) method. The calculated NMR parameters and simulated spectra suggest that Na + can be a good probe to investigate the layer structure. The spectra shape for 23 Na is more sensitive to isomorphic substitutions in the layer than the other nuclei.
Applied Clay Science, Dec 1, 2018
Many important properties of the clay minerals such as montmorillonite (Mt) arise from their surf... more Many important properties of the clay minerals such as montmorillonite (Mt) arise from their surface chemistry especially when this material is submitted to an acid activation process which will result in materials catalytically active for a variety of reactions. The acid activation preferentially occurs on the edge surfaces, in this sense ab initio calculations were performed in order to determinate the most probable models for (010) and (110) surfaces in both pyrophyllite and Mt clay minerals, the edge stability, the initial stages of the acid activation process and the main acid sites present in the structures. The calculations predicted that the (110) edge surface is more stable than the (010) edge surface at pyrophyllite, on the other hand, at Mt the (010) surface has shown to be more stable, thus the pyrophyllite edge surfaces should not be used as representative models of Mt edge surfaces. The microstructural evolution of the Mt during the acid activation was investigated by 29 Si and 27 Al SSNMR simulations and these calculations could assist in the correct assignment of the silicon and aluminum chemical environments. The reactivity of the surfaces was evaluated and the calculations indicate that the acid character of the acid-activated Mt mainly comes from the H sites present in silanol and aluminol groups. These sites might be responsible for the catalytic activity of these materials.
Applied Clay Science, Jul 1, 2017
Pentoxifylline (PTX) inclusion complex with β-cyclodextrin (β-CD) was investigated by theoretical... more Pentoxifylline (PTX) inclusion complex with β-cyclodextrin (β-CD) was investigated by theoretical and experimental methods. Based on the PTX electronic absorption band variation in presence of β-CD and using nuclear magnetic resonance experiments inclusion was verified. Data were confirmed by theoretical UV-Vis by means of TD-DFT calculations, demonstrating that β-CD cavity was able to reduce the oscillator strength of some electronic transitions, and indicating a close range between internal β-CD hydrogens to aliphatic PTX chain. Isothermal titration calorimetry experiments were useful to indicate that inclusion was spontaneous in solution. Additionally, at 298.15 K this inclusion process is enthalpic and entropic driven.
Acta Crystallographica, Aug 5, 2014
Layered double hydroxides (LDHs) or hydrotalite-like compounds belong to a class of synthetic two... more Layered double hydroxides (LDHs) or hydrotalite-like compounds belong to a class of synthetic two-dimensional inorganic materials with lamellar structures, where the hydroxyl-hydrated compounds are formed by chemical substitution of divalent ion of the brucitelike octahedral layers by trivalent ions [1]. The LDH are represented by the general formula, Figure a , where M2+, M3+ are divalent and trivalent cation and Am-is interlayer anion responsible by charge balancing, the range of divalent and trivalent ions (x) varies normally between 0.17 a 0.33. According Montanari and co-works [2] compounds type ZnAl-hydrotalcites (ZnAl-HT) have relevant industrial interest; however the scientific literature concerning this material as catalysts or catalyst precursor is scarce. Based on this fact, in the present communication we will present a studied on how the variation in percentage of Zinc in a ZnMgAl-HT catalyst precursor can influence the formation of mixed oxides after calcination. The ZnMgAl-HTs, Figure b , y=5, 10,15,20,25,50,75 and 100% were prepared by co-precipitation and urea method. Mg-Zn/Al mixed oxides were prepared from calcinations of hydrotalcites precursors at 500°C for 4 hours. The material synthesized were characterized by X-ray powder diffraction, the measurements were carried out in Bruker D8 DaVinci diffractometer, equipped with CuKα radiation , LynxEye linear Position Sensitive Detector, Ni-filter. Data was collect between 8 and 80° in 2θ with step size of 0.02° and the count time of 0.05 per step. Soller slit 2.5° of divergence and 0.2 mm primary slit were used. For the ZnMgAl-HT samples the measurements were performed at different temperatures, range 25-1200°C, heating rate 5°C/min. It was observed differences among XRD patterns for y greater than 25% of Zn in urea method at 500°C, and for co-precipitation method just for the substitution at 50 and 75% of Zn. These results suggested that the increase in Zn percentage change the structure of calcinated samples.
Recent Developments in Atomic Force Microscopy and Raman Spectroscopy for Materials Characterization [Working Title], 2021
This chapter covers a brief review of the definition, structural characteristics and main applica... more This chapter covers a brief review of the definition, structural characteristics and main applications of hydrotalcite, an interesting multifunctional material which finds applicability in different areas. Particularly, some catalytic reactions using hydrotalcite or mixed oxides derived from these materials are addressed (Ethanol Steam Reforming, Photochemical conversions, Hydrodesulfurization). The use of Raman Spectroscopy associated with other techniques, such as powder X-ray diffraction (XRD), Extended X-ray Absorption Fine-Structure (EXAFS), Temperature Programmed Reduction of hydrogen (H2-TPR), Fourier-Transform Infrared (FTIR) and Density Functional Theory (DFT) simulations, to characterize this type of material is addressed through examples described in the current literature. In this sense, multidisciplinary efforts must be made in order to increase the understanding of the properties of these materials and the catalytic behavior in the most varied reactions.
The role of Zn 2+ dopants in the acid-basic catalysis on MgO(001) surface: Ab initio simulations of the dissociative chemisorption of R-O-R′ and R-S-R′ (R,R′=H, CH 3 , C 2 H 5 )
Surface Science, 2017
Abstract Ab initio calculations were performed to study the effect of the Zn2+ dopant on the reac... more Abstract Ab initio calculations were performed to study the effect of the Zn2+ dopant on the reactivity and the catalytic activity of the MgO(001) surface toward molecular adsorption and dissociation reactions of the H2O, H2S, CH3CH2OH, CH3CH2SH and CH3SCH3 molecules. The electronic analysis showed that Zn2+ cation increased the reactivity of the surface locally. All molecules dissociate on both surfaces except for water and ethanol which only dissociate on the MgO:Zn(001) surface, confirming the increased reactivity in this surface. The Δ G ° for the dissociation reactions of the CH3CH2SH and CH3SCH3 molecules on pure MgO(001) surface is positive in the entire temperature range. On the other hand, the Δ G ° for H2S molecule is negative until 148.7 °C. In the case of the MgO:Zn(001) surface, the CH3CH2SH molecule dissociates in the entire temperature range and, for H2S molecule, the dissociation is spontaneous until 349.7 °C. The rate constants obtained for the dissociation reactions were very large because the reaction barriers are very low in both surfaces for all the studied molecules, except for CH3SCH3 molecule. The Zn-doped MgO(001) surface, besides being more reactive, presented a better catalytic activity than the MgO(001) surface for the dissociation of this molecule.
Acta Crystallographica Section A Foundations of Crystallography, 2011
Multiferroics have engendered increasing interest because of their many potential applications fo... more Multiferroics have engendered increasing interest because of their many potential applications for micro-or nano-electronic devices, magnetic storage elements and interesting fundamental physics. The term "multiferroic" means coexistence of ferroelectric and magnetic ordering in a single phase or multiphase material. However, the two ordering parameters are mutually exclusive because ferroelectricity and magnetism require different filling states of the d-shells of transition metal ions. Empty d-shells mainly exist in ferroelectricity, while partially filled d-shells are required in magnetism. Therefore multiferroics are rare. It exists several different microscopic mechanisms which may cause multiferroic behaviour. One of the most interesting is when a spontaneous polarization exists in a spiral or cycloidal magnetic structure. Accordingly, one strategy to find new multiferroic materials is to look for magnetic systems with that kind of magnetic structures. The complex metal oxides Mn 3 TeO 6 and Co 3 TeO 6 have been prepared both as single crystals by chemical transport reaction and as polycrystalline powders by a solid state reaction route. The crystal structure and magnetic properties have been investigated using a combination of x-ray and neutron powder diffraction, electron microscopy, calorimetric and magnetic measurements. It has been shown that at room temperature Mn 3 TeO 6 adopts a trigonal structure, space group R-3 (a= 8.8679(1)Å, c= 10.6727(2)Å) and Co 3 TeO 6 the monoclinic spacegroup C 2/c (a=14.7830(2)A, b=8.8395(1)A, c=10.3426(2)A). A long-range magnetically ordered state has been identified through variable temperature neutron diffraction and magnetic susceptibility measurements. The magnetic structure for the two compounds is very different. Mn 3 TeO 6 has an incommensurate helix structure while Co 3 TeO 6 shows a complicated but commensurate spin structure.
Acta Crystallographica Section A Foundations and Advances, 2014
Layered double hydroxides (LDHs) or hydrotalite-like compounds belong to a class of synthetic two... more Layered double hydroxides (LDHs) or hydrotalite-like compounds belong to a class of synthetic two-dimensional inorganic materials with lamellar structures, where the hydroxyl-hydrated compounds are formed by chemical substitution of divalent ion of the brucite-like octahedral layers by trivalent ions [1]. The LDH are represented by the general formula, Figure a , where M2+, M3+ are divalent and trivalent cation and Am- is interlayer anion responsible by charge balancing, the range of divalent and trivalent ions (x) varies normally between 0.17 a 0.33. According Montanari and co-works [2] compounds type ZnAl-hydrotalcites (ZnAl-HT) have relevant industrial interest; however the scientific literature concerning this material as catalysts or catalyst precursor is scarce. Based on this fact, in the present communication we will present a studied on how the variation in percentage of Zinc in a ZnMgAl-HT catalyst precursor can influence the formation of mixed oxides after calcination. The...
Applied Clay Science, 2015
In this work, the influence of the zinc content and synthesis methodology on the physicochemical ... more In this work, the influence of the zinc content and synthesis methodology on the physicochemical properties of mixed oxide derivatives from the hydrotalcite precursor MgZnAl-HT were evaluated. Layered double hydroxides (LDH) were prepared by co-precipitation, ultrasound and urea hydrolysis methods followed by calcination at 500°C. The materials were studied by such techniques as inductively coupled plasma, X-ray powder diffraction, Raman spectroscopy and 27 Al solid-state NMR spectroscopy. The textural and morphological properties of mixed oxides were evaluated through the adsorption/desorption of N 2 and scanning electron microscopy (SEM). Different textural and morphological properties were observed for the samples.
Structural comparison between samples of hydrotalcite-like materials obtained from different synthesis route
Química Nova
Three samples of hydrotalcite-like materials (HTC) were synthesized and their structural characte... more Three samples of hydrotalcite-like materials (HTC) were synthesized and their structural characteristics were compared with two HTCs obtained commercially. Thermal analyses, FT-IR, PXRD and textural analyses were used to investigate the structural differences between commercial and synthetic samples. Particularly, the memory effect was observed at temperature higher than 600 ºC. The Rietveld refinements were obtained with expressive accuracy and the statistical parameters of goodness of fit are quite satisfactory. In conclusion, the procedures adopted in synthesis of HTC produced crystalline materials with high surface area materials.
Materials Research Bulletin, 2015
Layered double hydroxides (LDHs) with Mg/Co/Al and Ni/Co/Al were synthesized for the first time b... more Layered double hydroxides (LDHs) with Mg/Co/Al and Ni/Co/Al were synthesized for the first time by the urea hydrolysis method. The experimental conditions promoted aluminum rich and crystalline materials. The formation of LDHs was investigated by powder X-ray diffraction (XRD), chemical analysis, solid state nuclear magnetic resonance with magic angle spinning (27 Al-MAS-NMR), simultaneous thermogravimetric/differential thermal analysis (TGA/DTA), FTIR spectroscopy, scanning electron microscopy (SEM), and N 2 adsorption-desorption experiments. A single phase corresponding to LDH could be obtained in all the investigated compositions. Thermal calcination of these LDHs at 500 °C resulted in the formation of solid solutions in which Al 3+ was dissolved. All the calcined materials have rock-salt like structures and high surface areas.
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Papers by Alexandre Leitão