Average Structural Parameters Research Articles

Studies on the evolution of asphaltene structure during hydroconversion of petroleum residues

The evolution of asphaltenes has been studied under hydroconversion conditions with an ebullated bed on a Buzurgan (Middle East) feedstock. A bench unit was used to produce effluents in residue conversion conditions ranging from 55 to 85 wt.%. In those conditions, asphaltene conversion ranged 62–89 wt.%. Asphaltenes from the feedstock and unconverted asphaltenes have been characterized using Size Exclusion Chromatography to evaluate asphaltene size, and 13C Nuclear Magnetic Resonance to evaluate the evolution of the average molecular structure parameters of asphaltenes. Results obtained were compared to results obtained at moderate residue conversion levels in a fixed bed. The work clearly shows that unconverted asphaltene evolution is continuous. Unit size decreases with increasing conversion while the aromaticity of the asphaltenes increases due to dealkylation. Our results suggest that the remaining asphaltenes are dissociated in smaller aggregates around 50% conversion. Below this level, asphaltene evolution may be related to dissociation mechanisms. Above 50% conversion, the chemical structure of asphaltene units still significantly evolves through dealkylation mechanisms, leading to fairly condensed structures that remain at high conversion. Based on these results, an attempt was conducted to interpret the evolution of unconverted asphaltenes as a function of residue conversion level using a simple molecular reconstruction method.

Effect of ethanol on the chemical structure of the soot extractable material of an ethylene inverse diffusion flame

The effect of fuel-side ethanol addition on the chemical structure of the soot extractable material generated in an ethylene inverse diffusion flame was evaluated by means of average structural parameters. The results indicate that the ethanol effect on the aromatic components was more pronounced, with an increase of about 40% in the average number of aromatic fused rings ( R a ) as compared to the results of a neat flame. This observation also helps explain the low percentage of chloroform-extractable material in the soot samples obtained from the flame with ethanol addition. In contrast, the aliphatic component of the extractable material did not demonstrate significant changes with ethanol addition.

Characterization of Asphaltenes from Hydrotreated Products by SEC, LDMS, MALDI, NMR, and XRD

Maya crude was hydrotreated at different reaction conditions in a pilot plant. Asphaltenes were precipitated from hydrotreated products with n-heptane as a solvent in a pressurized system with heating and stirring. Structural characterization studies have been carried out on asphaltenes from Maya crude. Molecular-weight distributions were obtained by size-exclusion chromatography (SEC) using tetrahydrofuran (THF) and 1-methyl-2-pyrrolidinone (NMP) as solvents. Nonfragmenting techniques, such as laser desorption mass spectrometry (LDMS) and matrix-assisted laser desorption/ionization mass spectrometry (MALDI−MS), were also used for determining molecular-weight distributions. Crystallite parameters of asphaltenes were studied by X-ray diffraction (XRD) after they were precipitated. Average structural parameters were calculated from nuclear magnetic resonance (NMR) spectra. The influence of reaction conditions are studied in terms of structural changes in asphaltenes.

Theoretical analysis of the effects of guanine oxidative damage on the properties of B-DNA telomere fragments

The molecular dynamics as well as ab initio MP2/6-31G(d = 0.25) single point calculations were performed for native and oxidized B-DNA telomeric fragments. The structural, dynamic, energetic and electrostatic properties along with frontier orbitals distribution were described for the central triad consisting of three guanine molecules in its canonical or oxidized forms. Although the average structural parameters characterizing all of the studied telomere fragments are close each to other, the significant consequence on angular and displacement flexibilities are observed. Namely, the increase of mutual displacement of two successive base pairs along either axis and increase of the rotation of two bases of opposite strand are main dynamic consequences of presence of 8-oxo-guanine in the central triad of telomeric B-DNA. Besides, the significant increase of stacking energies in case of 8-oxo-guanine were found. Furthermore, the guanine pattern visible from the major groove may be described as donor-acceptor-acceptor formed by H8-N7-O6 atoms, respectively. To the contrary the presence of 8-oxo-guanine changes the electrostatic properties of the major groove into acceptor-donor-acceptor coming from O8-H7-O6 atoms. This results in significant alteration of ESP characteristics. Finally, the molecular orbital properties are also significantly affected by oxidation of telomeric B-DNA fragments. All these factors contribute to decrease of binding of telomere proteins.

Local atomic structure in iron copper binary alloys: An extended X-ray absorption fine structure study

Understanding the clustering process and the evolution of the precipitate structure of copper in Fe–Cu system is an important step in the description of material hardening and the embrittlement process normally observed under irradiation conditions. In this work, an extended X-ray absorption fine structure (EXAFS) spectroscopy characterization of Fe–0.3wt%Cu and Fe–1.0wt%Cu binary model alloys has been performed in order to investigate the local structure around Cu and Fe atoms in the matrix. The effect of thermal ageing was studied on one Fe–Cu specimen containing 1.0wt% Cu annealed at 775K for a duration of 1h. The near-neighbour environment of Cu and Fe was examined by determining the best-fit structural parameters after curve fitting to the first-few-shells EXAFS functions. The results provide an indication of the formation of sub-nanometer-size Cu clusters in Fe matrix for the specimens examined. The average structural parameters estimated from the EXAFS data are presented and discussed.

Average structural analysis of the extractable material of young soot gathered in an ethylene inverse diffusion flame

Conventional analytical methods such as 1 H NMR, vapor pressure osmometry (VPO) and elemental analysis were used to characterize the soot precursor material represented by the chloroform extractable fractions of the young soot gathered at different heights of an ethylene inverse diffusion flame in terms of average structural parameters. The results indicate that the soot soluble fraction obtained at a 6 mm height has a relatively large molecular weight and has long aliphatic chains which later disappear with an increase in height above the burner base, especially in the region where the temperature is high (1200 K). This behavior is also accompanied by an increase in the aromaticity (fa) of the samples. 2006 Elsevier Ltd. All rights reserved.

Effect of n-Pentane and n-Heptane Insolubles on the Pyrolysis of Vacuum Residue

Tests have been performed using a vacuum residue from the Middle East and its subfractions, pentane solubles (C5S), pentane insoluble−hepane solubles (C5IC7S), and heptane insolubles (C7I), as feedstock in different solvents, to evaluate their ability upon coke formation. The feedstock including those three subfractions produced a negligible amount of coke (1−1.2 wt %) and 14.7−16.7 wt % asphaltene; the latter was formed more in the feedstock (9.4 wt %), at 420 °C. There was no significant effect on coke and asphaltene formations in the products depending upon the solvent used in the tests. Heptane-soluble subfraction (C7S) formed 9.2−13.6 wt % asphaltenes but no coke at 420 °C using 1-methylnaphthalene and decahydronaphthalene, while the asphaltene contents were only 2.4−6.0 wt % in the case of C5S. The properties and average molecular structural parameters indicated that C5S included only 5.1 aromatic rings per average fused ring system, while C5IC7S and C7I contained 8.3 and 10.1 aromatic rings, respec...

Characterization of Liquid Products Obtained from Cocracking of Petroleum Vacuum Residue with Plastics

Coprocessing of the petroleum vacuum residue (XVR) with polystyrene, polypropylene, and Bakelite has been undertaken in a batch reactor under isothermal conditions at atmospheric pressure. The liquids obtained by coprocessing have been characterized by Fourier transform infrared spectroscopy, 1H nuclear magnetic resonance (NMR), 13C NMR, gel permeation chromatography (GPC), inductively coupled argon plasma, and gas chromatography analyses. The main aim of the characterization of the liquid products was to find out the molecular weight distributions as well as the determination of average structural parameters. The GPC analysis showed that the liquids derived from XVR are quite complex, containing a complex range of products. The liquid products obtained from coprocessing showed that there is an interaction of reacting species when they are cracked together. It is also interesting that the liquid products obtained from coprocessing with plastics contained less than 1 ppm of Ni and V. The detailed results obtained are reported.

Evolution of Asphaltene Structure during Hydroconversion Conditions

The evolution of asphaltenes has been studied under hydroconversion conditions with an ebullated bed on a Buzurgan (Middle Eastern) feedstock. A bench unit was used to produce effluents in residue conversion conditions in the range of 55−85 wt %. Under those conditions, asphaltene conversion was observed in the range of 62−89 wt %. Asphaltenes from the feedstock and unconverted asphaltenes were then characterized using size exclusion chromatography (to evaluate asphaltene size) and 13C nuclear magnetic resonance (to evaluate the evolution of the average molecular structure parameters of asphaltenes). The work clearly shows that (i) the asphaltene unit size decreases when conversion increases and (ii) the aromaticity of asphaltenes increases due to dealkylation.

Applications of13C NMR to predict low temperature viscosity performance of base oils

Average structural parameters as determined by 13C nuclear magnetic resonance (NMR) spectroscopy, coupled with average base oil carbon number values from mass spectroscopy, were used to predict low temperature base oil viscosity performance. The advantages of this technique are that it provides quantitative carbon type distribution without any need for prior sample separation and can differentiate between various normal and iso-paraffinic structures. The 13C NMR technique also provides detailed information on the nature of branching in the iso-paraffinic hydrocarbons associated with waxy base oil components. Correlations between 13C NMR results and ASTM D 2983 Brookfield viscosities are reported for API Group II and Group III base oils, with R2 (adjusted) coefficients of 0.96 and 0.94, respectively. Results from this study also provide an indication of how specific base oil compositional features may affect low temperature viscosity performance. Copyright © 2006 John Wiley & Sons, Ltd.

Nominal doping and partition of doped holes between planar and apical orbitals inLa2−xSrxCuO4

By considering both the average structural parameters obtained from Rietveld refinement of neutron powder diffraction data, and the local structural parameters obtained from the atomic pair distribution function, we have tested the recent hypothesis of Perry et al. [Phys. Rev. B 65, 144501 (2002)] that doping in a ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ system occurs as localized defects of predominantly $\mathrm{Cu}{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}\text{\ensuremath{-}}\mathrm{O}{p}_{z}$ character associated with the Sr dopants accompanied by a local destruction of the Jahn-Teller distortion. While the structural parameters behave qualitatively according to the prediction of this model, a quantitative analysis indicates that doped holes predominantly appear in the planar $\mathrm{Cu}{d}_{{x}^{2}\ensuremath{-}{y}^{2}}\text{\ensuremath{-}}\mathrm{O}{p}_{x,y}$ band as is normally assumed. However, a small amount of the doped charge does enter the $\mathrm{Cu}{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}\text{\ensuremath{-}}\mathrm{O}{p}_{z}$ orbitals and this should be taken into account when theoretical phase diagrams are compared to experiment. We present a calibration curve, $p=x[1.00(1)\ensuremath{-}0.45(7)x]$, for the planar charge doping, $p$, vs strontium content, $x$, for the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ system.

Hyperfine electric field gradient tensors at Fe2+ sites in octahedral layers: Toward understanding oriented single-crystal Mossbauer spectroscopy measurements of micas

In this first systematic, theoretical study of the complete electric field gradient (EFG) tensor of 5 7 Fe Mossbauer spectroscopy as a function of chemistry and local structural distortion using electronic structure calculations, local EFG tensors were calculated at the central Fe 2 + sites in clusters of seven octahedra, where the neighbor octahedra were populated with a mixture of Mg 2 + and Al 3 + cations. The independent parameters of the EFG tensor-particularly the principal value V z z , the asymmetry parameter η, and the direction of the principal axis with regards to the octahedral sheet-were examined in detail as functions of octahedral flattening for each of the thirteen possible configurations of cations. We demonstrate that the argument that the EFG tensor at a particular site is largely determined by the point group symmetry of the corresponding crystallographic site is not correct. We find that in clusters with monoclinic or triclinic local point group symmetry, the value of V z z changes discontinuously from positive to negative as flattening increases. The principal axis changes from being in the plane of the octahedral sheet to being normal to it at this discontinuity, and the value of η reaches a maximum and begins to decline. This discontinuous behavior is caused by the continuous change of the EFG eigenvalues as the Fe(3d) character of the highest occupied spin-down orbital evolves with flattening. Taking EFG tensor results from all clusters, and using probabilities for the occurrence of each possible cation configuration in a chemically disordered octahedral sheet of a given bulk composition, we simulate averages and distributions of η, principal axis angles, and quadrupole splittings. While the value of η is broadly distributed from zero to one at all flattening angles and bulk compositions, there are two distinct populations of principal axes, normal to and in-plane with the octahedral sheet, as has been observed experimentally. We find these in-plane and normal populations of principal axes correspond exactly to populations with positive and negative values of V z z , respectively. Histograms representing quadrupole splitting distributions (QSDs) show features found in experimental QSDs, such as a constant high edge, a variable low edge, and a QSD width that changes dramatically with flattening. These results represent the first predictions relating average structural parameters, as would be obtained by X-ray diffraction, to characteristics of the Fe 2 + QSD, as obtained by 5 7 Fe Mossbauer spectroscopy, in a chemically disordered material.

Molecular dynamics simulation of liquid ethylene glycol and its aqueous solution

Molecular dynamics simulations were used to study pure liquid ethylene glycol (EG) and EG-water binary mixtures as a function of the molar fraction. Calculations were performed in the isothermal and isobaric (NpT) ensemble. New improvement in the OPLSS-AA-SEI force field was obtained optimizing the Lennard-Jones parameters and charges taking part in 1–4 interactions. With the new force field density and heat of vaporization of pure liquid ethylene glycol were obtained in good agreement with experimental data. The value of the O-C-C-O dihedral was also monitored in the course of force field optimization process. The new optimized force field was named OPLSA-AA-SE-M. Concerning the pure liquid simulation, features observed on the radial distribution functions (RDF) show characteristics of intermolecular and intramolecular hydrogen bonding. The RDF integrations show that each EG molecule in the pure liquid has an average coordination number of five molecules in its first coordination shell. Therefore, the average oxygen-hydrogen contact number due to hydrogen bonding is seven: two of them arising from intramolecular and five from interactions with neighboring molecules. Dihedral angle distributions were calculated, showing the predominance of EG molecules with the O-C-C-O dihedral corresponding to gauche conformations. In the study of water-ethylene glycol binary mixtures, the average structural parameters observed for EG molecules are very similar to the ones found in the pure liquid simulation. RDF analyses also present characteristic features of hydrogen bonding. Particularly, no significant changes were observed on the dihedral angle distributions. The structures of EG-EG and EG-water dimers calculated with OPLS-AA-SEI-M force field and DFT at B3LYP 6-311 g (3df,3pd) level are in very good agreement.

Compositional Analysis of Waxy Distillate Fractions by 1H and 13C NMR Spectroscopy

Distillate fractions having boiling ranges of 270–320°C and 320–370°C derived from Heera and Panna crudes are characterized for their average structural parameters using n-d-M method as well as 1h and 13C NMR spectroscopy. It is observed that these fractions contain predominantly saturated carbons. Aromatic carbons in them decreases with the increase in boiling range. Methyl substituents are predominant compared with other alkyl substituents in these distillate fractions.

Composition and Properties of Some Petroleum Waxes

Structural composition of paraffin waxes and soft wax fraction derived from microcrystalline wax were determined. Waxes were fractionated by multistage solvent crystallization at different temperatures. The n-alkane components of the waxes were separated by urea adduction. The average structural parameters of parent waxes, their fractions, and urea adductables were estimated by 1h and 13C NMR spectroscopy. The thermal parameters viz. phase transition temperature and the associated energy during phase transitions were determined by using DSC and correlated with the penetration temperature behavior of waxes. The carbon number distribution determined by GC for these waxes and their n-alkane components were also correlated with physical properties and thermal parameters.

Differences in Average Chemical Structures of Asphaltene Fractions Separated from Feed and Product Oils of a Mild Thermal Processing Reaction

The differences in the chemical structures of the asphaltene fractions present in a vacuum residue feed and in product oil after mild thermal processing, in the presence of N2 and H2 gases (activation gases), was studied using measurements of molecular weight, elemental analysis, nuclear magnetic resonance (NMR) spectroscopy, and X-ray diffraction (XRD). The asphaltene fractions were separated from the feed and product oil according to standard procedures. The percentage intensities of H and C nuclei in different chemical environments were obtained from 1H NMR and 13C NMR spectra of these fractions. The numbers of H atoms and C atoms in various building blocks were thus calculated. The average structural data for the feed and product asphaltene molecules were derived. The macrostructure and aromaticity parameters were determined using XRD and compared with the average structural parameters calculated from NMR. The data show that the aromatic ring structures were similar, although it was the aliphatic stru...

The Fourier-transform analysis of Ti K-XANES in metamicts and glasses

The structural parameters of Ti coordination environment in glassy titanite (CaTiSiO5), natisite (Na2TiSiO5) and radiation damaged CaTiSiO5 are determined by the method proposed earlier, based on the Fourier-transform analysis of Ti K-XANES over the short k-range of photoelectrons wave numbers. The accuracy of the determined parameters is tested for the model crystals CaTiSiO5, Na2TiSiO5. The modelling of the extracted contribution into Ti-XANES, arising from the oxygen-polyhedron around the ionised Ti atom in amorphous titanites and natisite, performed via the single-shell fit and the two-shells fit, returns not only the averaged structural parameters, but also reveals the main groups of the Ti–O distances for this polyhedron.

Structural properties of free-standing (In,Ga)(As,N) alloys

The role of average composition and local compositional fluctuations on the structural properties of Ga1−yInyAs1−xNx quaternary alloys and related ternary alloys was investigated by means of accurate density functional pseudopotential calculations. For the quaternary alloy, the authors focus particularly on the case with x=0.5 and y=0.5. They study the different anion–cation bond length distributions and the macroscopic average structural parameters.

Influence of Chemical Structures on Low-Temperature Rheology, Oxidative Stability, and Physical Properties of Group II and III Base Oils

The effect of base oil composition on lubricant properties has been studied to better understand low-temperature rheology, oxidative stability, and other physical properties. A simple approach has been adopted using low-temperature rheology to develop a multi-linear regression model to estimate Brookfield viscosity. Six base oils from API groups II and III were studied using different nuclear magnetic resonance spectroscopy methods. One method involved determination of average structural parameters from quantitative 13C NMR data, while the other method involved measurement of spin−lattice relaxation times at variable temperatures. The structural information obtained from these two methods was used to develop meaningful correlations for low-temperature, oxidative, and physical properties of these base oils. The study suggests that a deeper knowledge of chemical composition will help in selecting base oils to meet future product specifications.

Chemical composition of high resin petroleum tar

The data from physico-chemical methods (elemental analysis, IR, NMR spectroscopy, etc.) and integral structural analysis are used to calculate the average structural parameters and to determine structural-molecular models of compounds contained in commercial tar of West Siberian, Romashkino and Arlan petroleums. The high molar mass compounds of the petroleums examined were characterized by unique principles of structural-molecular organization of corresponding typical fragments, which include similar naphthene-aromatic condensed systems of five-to-six rings; the results explain the possibility of mutual transitions of high molar mass petroleum compounds as well as the katagenic transformations of petroleum during processing. 12 references, 8 tables.