N-heptane Molecules Research Articles

Effect of Pt and Sn on the adsorption of n-heptane in γ-Al2O3 catalyst models

The Grand Canonical Monte-Carlo (GCMC) method has been used to carry out simulations of the adsorption of n-heptane in models of naphtha-reforming catalysts. Models used in the study differed in the number and distribution of metal atoms-Pt and Sn. The number of adsorbed n-heptane molecules grows linearly with increasing number of metal atoms. The effect of Pt content on the adsorption of n-heptane molecules is most distinct at approximately 100 kPa and within the lower range of the temperatures investigated. In the models of bimetallic catalysts, the effect of the two metals is additive. [Figure: see text]. Effect of Pt and Sn on number of n-heptane molecules adsorbed in Al2O3 catalyst in 773 K and 1000 kPa.

Adsorption of petroleum organic compounds on natural Wyoming montmorillonite

Adsorption of petroleum organic compounds on natural Wyoming montmorillonite clay is an irreversible process, as most of organic species still remain adsorbed on mineral after aging. The process is conditioned by the petroleum paraffinic content and by molecules containing alkane chains, as only these type of compounds easy intercalate within the clay interlayer. Such adsorption also involves an irreversible displacement of part of the clay interlayer water. The partial dissolution of the clay matrix is indicated by XRD as an irreversible modification of the clay structure. On the contrary, adsorption of n-heptane or toluene molecules on the same clay are reversible processes, since the clay mineral recover its original crystalline structure after aging and evaporation of the organic species.

Molecular dynamics simulation of the coalescence of nanometer-sized water droplets in n-heptane.

Molecular dynamics simulations using a modified Drieding 2.21 force field were carried out to study the coalescence behavior of nanometer-sized water droplets in vacuum and in n-heptane. The coalescence mechanisms of the water droplets in the above-noted environments are fairly similar in a sense that the water droplets form a bridge linking the droplets before they merge. However, in the latter situation, due to the presence of n-heptane molecules in between the water droplets, the coalescence was observed to be slowed down considerably, especially in the first 10 ps of the process. However, once the bridge is formed, the water droplets, in both situations, spend about the same amount of time to form a single droplet. The maximum distance between the droplets above which coalescence does not occur was found to be 10 A. In terms of the dynamics, the diffusion coefficient of n-heptane in the emulsion system was very close to its value in the pure liquid form. This may be because n-heptane is the continuous phase. Nonetheless, the dynamic behavior of water in n-heptane is different from that of pure water during and after the coalescence. In particular, the self-diffusion coefficient of water molecules in n-heptane is about 20% higher than the experimental value of pure water. Due to the lack of strong attraction forces between water and n-heptane molecules, the n-heptane molecules were observed to orient themselves perpendicularly to the water/n-heptane interfaces so that the contacting area is minimized.

The effect of coke lay-down on n-heptane reforming on Pt and Pt-Sn catalysts

In this paper, we report detailed studies regarding the reforming of n-heptane and related molecules carried out under near-industrial conditions on supported Pt and Pt-Sn catalysts of the type used in industry. We have correlated the performance of these catalysts with the level of coke laid down on the surface measured by TGA. The results indicate that the presence of coke reduces hydrogenolysis/cracking and favours aromatisation. The presence of tin in the catalyst produces much faster coke lay-down, and, in turn, this gives higher selectivity to the desired products in a much shorter time on stream. The initial rate of coke lay-down is very rapid, but it slows to a near-zero rate after this initial period, when 2% of the catalyst weight of carbon has been deposited on the surface; we propose that this is a near-monolayer passivating layer which promotes toluene production. The presence of chlorine on the support increases the activity, the cracking and the coking. Consideration is given to the nature of the species responsible for the coking of the catalyst.

Determining the Adsorption Sites for Binary Mixtures of p-Xylene and n-Heptane in Silicalite Using FT-Raman Spectroscopy and Temperature-Programmed Desorption

FT-Raman spectroscopy and temperature-programmed desorption (TPD) measurements have been used to gain information on the preferred adsorption sites for p-xylene and n-heptane adsorbed within silicalite. FT-Raman spectroscopy is used to probe the location of adsorbed p-xylene, both when pure p-xylene is adsorbed and for the binary mixtures. The TPD measurements in combination with the FT-Raman spectra are used to infer the location of the adsorbed n-heptane. The results on the preferred adsorption sites for pure adsorbates are consistent with those in the literature. The results on binary mixtures show unusual behavior at loadings of four p-xylene/three n-heptane and three p-xylene/four n-heptane molecules per unit cell. In contrast to the behavior for single component adsorption, at these loadings the n-heptane molecules preferentially occupy the straight channels of silicalite, forcing the p-xylene molecules out of their normal preferred site (the channel intersections) and into the zigzag channels. The ...

Hydroconversion of hydrocarbons over Ru-containing supported catalysts prepared by metal vapor method

Some mono- and bimetallic-supported Ru/ZSM-5, Ru–Re/ZSM-5, Ru–Rh/ZSM-5, Ni–Ru/ZSM-5 and Ru–Ni/ZSM-5 catalysts were prepared by metal vapor method. The carbon–carbon cleavage activity and the selectivity of prepared catalysts were investigated in hydroconversion of alkanes ( n-pentane, n-hexane, n-heptane, 2-methylpentane) and cycloalkanes (cyclopentane, cyclohexane, methylcyclopentane). These catalysts demonstrate extremely high C–C bond splitting activity in tested hydrocarbons. The magnitude of the effect for methylcyclopentane reaches a factor of about 70 at 393 K in comparison with the activity of known Ru/TiO 2 at 433 K. The increase in activity for the metal vapor method prepared Ru-containing catalysts is due mainly to the presence of highly dispersed ruthenium particles on a zeolite support. The selectivity of Ru/ZSM-5 remains in a range expected for known well-dispersed Ru-containing catalysts, whereas these values can be modified to a large extent by addition of a second metal to the mono-metallic samples: Ru–Re/ZSM-5 shows high selectivity in the isomerization of n-hexane to 2- and 3-methylpentanes, while Ni–Ru/ZSM-5 ruptures mostly the central carbon–carbon bond in the n-heptane molecule. The rates of hydroconversion of tested hydrocarbons, based on a lodged ruthenium, on most of the bimetallic samples are much higher than over Ru/ZSM-5.

Molecular Structure and Phase Diagram of the Binary Mixture of n-Heptane and Supercritical Ethane: A Gibbs Ensemble Monte Carlo Study

Pressure-composition and temperature-composition phase diagrams are computed for the binary mixture of n-heptane and supercritical ethane using four different molecular models of increasing complexity. The ethane and heptane molecules were described using either (i) single-site Lennard−Jonesium (LJ) with fixed well-depth and size parameters, (ii) single-site LJ with temperature-dependent well depths, (iii) chains of methyl and methylene pseudo-atoms interacting via LJ potentials placed at the positions of all carbon nuclei, or (iv) an explicit-hydrogen representation with LJ interaction sites placed both at the carbon nuclei and at the centers of all carbon−hydrogen bonds. For comparison, simulations were also performed for the binary mixture of n-heptane and helium using the pseudo-atom model for the n-heptane molecules. All four models produce phase diagrams for the ethane/n-heptane mixture that are in reasonable agreement with experiment. However, the accuracy of the calculated phase diagrams improves markedly with increasing complexity of the model (and therefore increasing computational requirements). In both the liquid (n-heptane-rich, higher specific density) and supercritical (ethane-rich, lower specific density) phases center-of-mass radial distribution functions appear to show more enhanced structures for the two single-site models than the united-atom and explicit-hydrogen force fields. However, the number integrals of these radial distribution functions are strikingly similar for all models, that is the differences in molecular shape do not lead to a difference in the clustering of the solvent molecules around the solute. In particular, preferential solvation of n-heptane by ethane is not observed in the supercritical phase. Analysis of the contributions of the liquid and the supercritical phases to the decrease of the Gibbs free energy of transfer of n-heptane with increasing pressure suggest that the enhanced solubility of n-heptane in high-pressure supercritical ethane can be attributed to two causes of roughly equal importance: “Pulling” of n-heptane into the supercritical phase by an increased density of ethane that acts as a nonspecific solvent, and “pushing” n-heptane out of the liquid phase by an increased concentration of ethane.

Theoretical study of then-heptane-HZSM-5 ring structure model interaction

We present a theoretical analysis for the interaction of the n-heptane molecule with an HZSM-5 zeolite, modeled as a ring structure. The Turbomole program, which is a density functional theory based method, was used. Quantum mechanical (QM) calculations were all-electron using the gradient-corrected BLYP approach. We employed orbital basis sets of DZP quality for all atoms. Two coordination modes were studied for the n-heptane–zeolite interaction: a reference structure with the n-heptane moiety located at the center of the ring, and a structure where n-heptane is close either to a Broensted acid site (the region around the Al atom) or to a Lewis acid site. Although the chosen ring represents a minimal model for a zeolitic cavity, the obtained results give insight about the formation of the carbocationic species, proposed as intermediaries during the catalytic cracking reactions. The key electronic effects such as charge transfers and frontier molecular orbitals, involved in the adsorption of n-heptane over the inner surface of the HZSM-5 cavity are presented and discussed for the representative coordination modes studied. The Mulliken and the Roby-Davidson population analysis were done. They are very useful, particularly the second one, to identify the catalytic sites, nucleophilic and/or electrophilic centers, as well as to locate the possible intermediates or transition states with a carbocation character, which are of considerable importance in the hydrocarbon catalytic cracking chemistry. Lastly, we have studied some of the effects that the surroundings produce on the chosen rings–hydrocarbon systems. This was accomplished through the use of a QM/MM(molecular mechanics) methodology. In this way, the embedded QM region representing the cavity it is described more properly for these host–guest interactions. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 725–740, 1999

Monte Carlo Study of the Thermodynamic and Structural Properties of 1-Butanethiol + N-Heptane Mixtures

Abstract The ability to use computer simulations to predict mixture properties using potential models that have been optimized for the pure compounds is demonstrated. Since the potential models were optimized only for the pure compounds, there is no guarantee that they will describe the interactions between dissimilar molecules in the mixture correctly. In this study, Monte Carlo simulations have been carried out in the isothermal-isobaric (NPT) ensemble to calculate the density and excess enthalpy for 1-butanethiol + n-heptane mixtures at 298.15 K and 1 atm. The OPLS potential-parameters developed by Jorgensen were used to describe the n-heptane molecule. Two models for the butanethiol molecule were employed: PES1 used the OPLS potential-parameters unaltered, while PES2 used the OPLS parameters with slightly modified partial charges. Simulations were performed on mixtures with butyl mercaptan mole fractions of 0.0, 0.23, 0.42, 0.62, 0.83, and 1.0. The average rms deviation between the calculated densities for PES1 and PES2 and the experimental results is 0.021 g/cc and 0.015 g/cc, respectively, while the average rms deviation for the excess enthalpies for PES1 and PES2 is 0.058 kcal/mol and 0.027 kcal/mol, respectively. We also compared our calculated densities with the COSTALD correlation. The extent of self-association of the butanethiol molecules was found it to be small for all of the mixtures for the PES2 model.

Influence of a dielectric medium on the phase state of carbon dioxide

The phase state of carbon dioxide gas dissolved in liquid n-heptane is determined by experimental investigations of the temperature dependences (180 < T < 250°K) of the spin-lattice relaxation time of protons, the coefficient of translational self-diffusion of n-heptane molecules and the nuclear magnetic resonance (NMR) linewidth of carbon13C.

Infrared study of the adsorption of hydrocarbons on rutile at the solid–vapour and solid–liquid interfaces

The adsorption of n-heptane, toluene and 1,4-dimethylbenzene vapours on to the surface of rutile has been studied by infrared spectroscopy. Surface hydroxyl groups were perturbed to a lesser extent by weak interactions with physically adsorbed n-heptane molecules than by stronger hydrogen-bonding interactions with the aromatic nuclei of toluene or 1,4-dimethylbenzene. Hydroxyl groups responsible for an infrared band at 3410 cm–1 in spectra of rutile were unaffected by the adsorption of hydrocarbons and are thought to be at sub-surface lattice sites. Two types of surface hydroxyl group were replaced by a chemisorbed oxidation product, possibly acetate anions, when rutile was exposed to 1,4-dimethylbenzene vapour.

A relationship between the zeta potential and surface free energy changes of the sulfur/ n-heptane—water system

Measurements of the zeta potential of sulfur in doubly distilled water were carried out by the streaming potential technique. The zeta potential of sulfur changes from −25.6 to −120 mV if the sulfur surface is wetted with n-heptane. The relationship between the amount of heptane wetting the surface and the zeta potential has been determined. On this basis, the film pressure for heptane and then the dispersion part of surface free energy of sulfur have been determined. For comparison, similar measurements have been made with a Teflon (PTFE)/ n-hexane-water system. If the determined values of film pressure are treated as those resulting from immersional wetting, good values of dispersion energy are obtained, e.g., 97.6 and 19.7 ergs/cm 2 for sulfur and Teflon, respectively. It was assumed that n-heptane molecules are vertically oriented against the sulfur surface, at least in the first layer. This conclusion was verified theoretically by Fowkes' relation for calculation of the relative pressure, p P 0 for first monolayer. It has been assumed here that relative zeta potential correlates to p P 0 . The calculated relative zeta value is 0.28, and the experimental one is 0.26.

Pressure-induced changes in molecular conformation in liquid alkanes

LIQUID linear alkanes at room temperature and pressure exist in a mixture of molecular shapes. The n-heptane molecule, for example, has 13 possible distinct conformations with up to four gauche bonds distributed along its length. In the crystal, however, all the molecules assume the all-trans, straight chain shape. We have studied the conformations of chain molecules in the liquid state in various conditions using Raman scattering as a probe1. We have looked particularly at what happens to the shapes of linear alkane molecules subjected to high pressures in the liquid state. We speculated that as the pressure on a liquid of chain molecules was increased toward the freezing point that the chains might begin to straighten out. In fact, we found that an increase in pressure caused an increase in the number of gauche bonds, that is, pressure caused the molecules to become more globular.

Hydrocarbon radiolysis in the presence of carboranes(12) 3. Formation of product hydrocarbons

A study has been made of the effect of o-carborane on hydrocarbon formation in the radiolysis of n-heptane. The introduction of o-carborane into the n-heptane system reduced the yield of “intermediate” and dimeric hydrocarbon products. The inhibiting effect of o-carborane traces back to its electron-acceptor activity and its ability to deactivate the excited n-heptane molecule.

DISTRIBUTION OF DISPERSE DYES BETWEEN &lt;i&gt;n&lt;/i&gt;-HEPTANE AND CHLORINATED &lt;i&gt;n&lt;/i&gt;-HEPTANE

Isotherms were measured for distributions of model disperse dyes (benzene, azobenzene, p-amino-azobenzene, p-nitroazobenzene, 4-amino-4′-nitroazobenzene, 4-N-methylamino-4′-nitroazobenzene, and 4-N, N-dimethylamino-4′-nitroazobenzene) between water and n-heptane and between water and 3.5% chlorinated n-heptane at various temperatures. From the results the thermodynamic parameters were calculated for the process which consists of the transfer of one mole of the dye molecule from n-heptane to 3.5% chlorinated n-heptane.Dye in n-heptane (mole fraction) → Dye in 3.5% chlorinated n-heptane (mole fraction)The unitary free energy changes accompanied by the process were all negative. This indicates that the dye molecules were more stable in chlorinated n-heptane than in n-heptane. From the thermodynamic parameters obtained in this work the interaction between the dye molecule and chlorinated n-heptane molecule was discussed. It was concluded that the ability of the chlorine atom of chlorinated n-heptane to form hydrogen-bonds with proton-donating groups of the dye molecule might be a main source of the stability of the dye molecule in chlorinated n-heptane.The fact that an introduction of an amino group into the dye molecule caused a marked increase in enthalpy change supported this conclusion.