N-heptane Molecules Research Articles

Reversible solvent interactions with UiO-67 metal-organic frameworks.

The utility of UiO-67 Metal-Organic Frameworks (MOFs) for practical applications requires a comprehensive understanding of intermolecular host-guest MOF-analyte interactions. To investigate intermolecular interactions between UiO-67 MOFs and complex molecules, it is useful to evaluate the interactions with simple polar and non-polar analytes. This problem is approached by investigating the interactions of polar (acetone and isopropanol) and non-polar (n-heptane) molecules with functionalized UiO-67 MOFs via temperature programmed desorption mass spectrometry and temperature programmed Fourier transform infrared spectroscopy. We find that isopropanol, acetone, and n-heptane bind reversibly and non-destructively to UiO-67 MOFs, where MOF and analyte functionality influence relative binding strengths (n-heptane ≈ isopropanol > acetone). During heating, all three analytes diffuse into the internal pore environment and directly interact with the μ3-OH groups located within the tetrahedral pores, evidenced by the IR response of ν(μ3-OH). We observe nonlinear changes in the infrared cross sections of the ν(CH) modes of acetone, isopropanol, and n-heptane following diffusion into UiO-67. Similarly, acetone's ν(C=O) infrared cross section increases dramatically when diffused into UiO-67. Ultimately, this in situ investigation provides insights into how individual molecular functional groups interact with UiO MOFs and enables a foundation where MOF interactions with complex molecular systems can be evaluated.

ReaxFF molecular dynamics simulation on the combustion mechanism of toluene/ethanol/n-heptane mixed fuel

The addition of ethanol and toluene to diesel fuel serves as an effective approach to mitigate engine knock. In this work, we employed ReaxFF molecular dynamics simulations to investigate the oxidation of toluene, ethanol, and n-heptane mixed fuels. Results show that toluene, ethanol, and 60.12 % of n-heptane molecules initiate dehydrogenation via active species such as O2, OH, HO2, etc. The initial decomposition mechanisms of the remaining n-heptane molecules involve the fission of CC bonds and direct dehydrogenation reactions. The formation of intermediate C2H4 primarily occurs via β-cleavage of saturated alkyl radicals with higher carbon atom numbers. The products CO and CO2 molecules strongly depend on the concentrations of C2H4 and CH3. Notably, the incorporation of ethanol/toluene in the system diminishes the presence of OH, thereby decreasing the consumption of C2H4 by OH. The activation energy values (43.36 ∼ 45.19 kcal mol−1) align well with experimental data.

Synthesis, crystal structure, and sensing properties of organic molecules based on a Terbium-nitronyl nitroxide complex

A new Terbium-nitronyl nitroxide radical complex [Tb(hfac)3(NITPh-m-CHO)2]·0.5C7H16 (Tb-rad) (hfac = hexafluoroacetylacetonate; NITPh-m-CHO = 2-(3′-formylphenyl)-4,4,5,5-tetramethylimidazoline-l-oxyl-3-oxide) was prepared and characterized. Tb-rad crystallizes in triclinic space group P1¯, possessing a mononuclear structure, in which the geometrical configuration of Tb(III) is triangular dodecahedron consisted of six oxygen atoms from three hfac ligands and two oxygen atoms from the NO groups of two NITPh-m-CHO ligands. In Tb-rad, isolated mononuclear structures were connected to form 1D supramolecular chain structures via intermolecular π···π interaction and free n-heptane molecules fill the void in the crystal. Significantly, the luminescence explorations demonstrate that Tb-rad is an excellent fluorescent sensor for identifying nitrobenzene (NB) and nitromethane (NM) molecules. Because of the linear relationship at low concentration with the quenching constant (Ksv) is 5.4 × 105 and 6.6 × 103 M−1 for NB and NM, respectively, it can be considered as a potential luminescent sensor for detecting NB and NM molecules. Moreover, the possible sensing mechanisms of selective detection have been discussed in detail.

Molecular Dynamics Simulations of Vapor-Liquid Interface Properties of n-Heptane/Nitrogen at Subcritical and Transcritical Conditions.

Vapor-liquid interfacial properties of n-heptane/nitrogen at various temperatures and pressures in the ranges of 270-648 K and 10-60 bar were investigated using molecular dynamics (MD) simulations. Additionally, density gradient theory (DGT) was used to predict the binary system interface for qualitative comparison purposes. Results show that surface tension decreases with the increases in both temperature and pressure, but the rate of decrease against the latter is smaller. The interface thickness, which is of the order of several nanometers, was also shown to have a much stronger dependence on temperature and almost invariant with respect to pressure. MD also reveals the phenomenon of nitrogen enrichment in the outer interfacial region at low subcritical temperatures. A sensitivity analysis of influence parameters on DGT results revealed that surface tension values are a stronger function of n-heptane influence parameter and almost invariant with respect to nitrogen influence parameter. The velocity distribution function (VDF) shows that the molecules in and away from the interface follow the Maxwellian distribution of kinetic theory at subcritical conditions. The VDF of the velocity component normal to the interface of the evaporated n-heptane molecules adjacent to the interface exhibits a deviation from the Maxwellian distribution. The VDF of all three components of velocity of n-heptane molecules at supercritical conditions exhibits a departure from the Maxwellian distribution as well.

1D Modeling of the Microwave Discharge in Liquid n-Heptane Including Production of Carbonaceous Particles

Microwave plasma in the liquid is initiated inside a gas bubble formed at the end of the electrode-antenna, through which microwave energy is introduced into the liquid. A 1D model a set of gas phase kinetic reactions describes the evolution processes of ionization, heat transfer and formation of gas and solid products inside the plasma bubble. The code is based on joint solution of the Boltzmann equation for free electrons of the plasma, a simplified equation for the microwave field, the heat conduction equation, the balance equation for the electron density and the balance equations for the weight fraction for all gas and solid products of n-heptane pyrolysis. The Joule heat released in the plasma is expended on the evaporation of liquid n-heptane into the bubble and the decomposition of the n-heptane molecules. The model includes both the description of gas phase processes and formation of solid carbon-containing particles. The growth mechanism for generation of solid particles describes simultaneous processes of the initial nucleation, surface growth and coagulation of soot particles. The results of calculations are compared with known experimental results.

Molecular conformational effects in H + n-heptane reaction rate calculations

Accurate fuel combustion modelling is a matter of immense importance to design clean combustors and reduce greenhouse gas emissions and pollutants. In this Brief Communication, we present the effects of internal dynamics of one n-heptane molecule which are controlling chemical kinetics of hydrogen abstraction reactions through multi-pathway reaction dynamics. It is established that the slope of Arrhenius plots dramatically changes in comparison with the harmonic single static pathway approach in the temperature range of 200–3000 K. We apply a combination of the multiple conformation statistical thermodynamic approach and variational transition-state theory (VTST) to obtain dynamic multi-path rate coefficients (kMP-T-VTST and kMP-LH-VTST). Compared with single-path VTST (kSP-H-VTST) results, the thermal reaction rate coefficients obtained from our MP-VTST calculations differ considerably due to the fact that tunnelling and cross-conformational effects in the reactions, and the anharmonic and quasi-harmonic contributions in multiple conformer molecules cannot be ignored or simplified.

Positron insight into evolution of pore volume and penetration of the polymer network by n-heptane molecules in mesoporous XAD4.

The adsorption and desorption of n-heptane on the mesoporous polymer resin Amberlite XAD4 were investigated in situ by positron annihilation lifetime spectroscopy (PALS). This technique allows the monitoring of porosity and subnanometer free volume changes as well as the amount of liquid adsorbate captured within an investigated sorbent, without causing any interference with the course of adsorption/desorption. In consequence, the conducted studies provide microscale insight into the sorption processes of n-heptane (which is a significant component of volatile organic compounds - VOCs) on the polymeric material. The total pore volume decreases parabolically with n-heptane pressure until it reaches zero just below the saturated vapor pressure. Simultaneously, the average pore size increases linearly until it has approximately doubled. However, much faster rates of change in both these parameters occur at relative pressures below 0.05. The PALS results can be properly explained only if the swelling of the polymer skeleton is taken into account during the alkane adsorption process. This is confirmed by long-term pumping, which was required to achieve stabilization of PAL spectra during the final phase of desorption. In addition, the evolution of subnanometer free volumes (located between polymer chains and formed in liquid n-heptane) support this interpretation of the results.

Visualization of asphaltene precipitation and deposition in a uniformly patterned glass micromodel

Precipitation and deposition of asphaltene are among chronic operational problems for recovering oil from reservoirs of high asphaltene content. The present experimental study aimed at investigating the process of precipitation and deposition of asphaltene in a uniformly patterned glass micromodel. This was carried out by the injection of n-heptane while setting the other operating conditions, such as temperature, pressure and flow rate unchanged. Synthetic oil solutions containing toluene and n-heptane with asphaltene content of 5wt% were prepared and injected to the micromodel under different wettability conditions. Precipitation of asphaltene gradually started once n-heptane was introduced to the micromodel. It was found that asphaltene precipitation occurred as injected n-heptane diffused into the oil phase and destabilized the dissolved asphaltene. The destabilization of dissolved asphaltene occurred along with the formation of a cloud-like, semi-solid phase which evolved from the oil phase due to the diffusion of n-heptane molecules indicating the onset of asphaltene precipitation. As soon as precipitation took place, the color of oil phase changed from black to grey due to loss of dissolved dark asphaltenes. Asphaltene precipitation in the oil-flooded micromodel was more likely to occur due to the deposition of asphaltene onto the micromodel surface and passages. Finally, it was found that not all precipitated asphaltene particles would deposit, instead it was more likely occurrence (i) for the oil-flooded micromodel; (ii) micromodels passages with tortuosity and complicated flow paths which have been generated by the deposition of precipitated asphaltenes and (iii) the interface of n-heptane and the oil phase.

2H and 13C NMR studies of orientational order and actual amount of n-heptane molecule in liquid crystal phases of the 7CB–n-heptane system

The orientational order of n-heptane molecules in the nematic phase and the smectic A (SmA) phase of the 4-heptyl-4′-cyanobiphenyl (7CB)–n-heptane binary system was investigated by 2H NMR spectroscopy. The increase in the mobility of the alkyl chain region corresponding to an increase in the amount of n-heptane enhanced the micro-segregation of the core and the alkyl chain region and contributed to the formation of the SmA phase for this system. The actual amount of n-heptane molecule adsorbed in the SmA phase was assessed by 2H and 13C NMR for the coexistence state of the isotropic liquid and the SmA phase.

Thermodynamic and Modeling Study of Thin n-Heptane Films Adsorbed on Magnesium Oxide (100) Surfaces

Thermodynamic properties of n-heptane adsorbed on the MgO(100) surface were investigated using high-resolution, volumetric adsorption isotherms in the temperature range of 205–275 K. Two distinct molecular layers were observed in all isotherms. The heat, differential enthalpy, differential entropy, and isosteric heat of adsorption were determined. Using the temperature dependence of the two-dimensional compressibility, two phase transitions were observed at 246.0 ± 2.0 K and 249.3 ± 1.2 K for the first and second layers, respectively. The average area per n-heptane molecule adsorbed on MgO was estimated to be 99 ± 10 A2, suggesting that the carbon backbone is preferentially oriented parallel to the surface. The COMPASS force field was used to calculate the minimum energy configurations of an n-heptane molecule on the MgO(100) surface. The calculations support experimental evidence of the carbon backbone oriented parallel to the surface and additionally suggest a preferential alignment of the molecule alon...

J0550305 イソオクタンならびにn-ヘプタン分子の運動エネルギー解析 : 並進・回転・内部振動・コリオリ運動([J055-03]マイクロ・ナノスケール熱流体現象(3))

J0550305 イソオクタンならびにn-ヘプタン分子の運動エネルギー解析 : 並進・回転・内部振動・コリオリ運動([J055-03]マイクロ・ナノスケール熱流体現象(3))

Kinetic Modeling of the Free-Radical Process during the Initiated Thermal Cracking of Normal Alkanes with 1-Nitropropane as an Initiator

To elucidate the accelerating effect of 1-nitropropane on the thermal cracking of normal alkanes, ab initio and density functional theory calculations were performed to investigate the elementary reactions involved in the initiated thermal cracking of a 1-nitropropane/n-heptane mixture. The kinetic parameters were evaluated on the basis of standard transition state theory (TST) or variational transition state theory (VTST) with Wigner tunneling correction. The activation energy for the C–N bond rupture of 1-nitropropane to produce primary n-propyl and nitro radicals is calculated to be 234–269 kJ/mol, while the bond dissociation energy of the C–C bond within the n-heptane molecule is predicted to be at least 335 kJ/mol. These calculated results demonstrate that the presence of 1-nitropropane makes the free radical formation become relatively easier compared with single n-heptane cracking. Furthermore, compared with the C–H bond cleavage and 1,3-H intramolecular transfer, the unstable n-propyl radical main...

A Skeletal Chemical Kinetic Model of PRF Oxidation for HCCI Engines

A skeletal chemical kinetic mechanism, including 42 species and 71 reactions for the oxidation of primary reference fuel (PRF), was developed and applied to model homogeneous charge compression ignition (HCCI) combustion after various experiments and available mechanisms for PRF oxidation being reviewed and the performance of mechanisms applied to experiments relevant to HCCI engines being analyzed. The ignition delay predicted by the skeletal mechanism showed good agreement with experiments for different fuels over the temperature range 667-1350K. Validation was also performed with experiments from HCCI engines, and good agreement was obtained for the primary reference fuels. The results show that the present PRF mechanism gives reliable performance for HCCI combustion predictions. Sensitivity analysis indicates that for PRF70 and PRF91.8 under HCCI conditions, H-atom abstraction from iso-octane molecule reaction and OH addition to n-heptane molecule have large influence, and CH2O, CH3 are very important intermediate species.

Surface Electronic Structure–Catalytic Activity of Different Mo Oxidation States for Olefins and Saturated Hydrocarbon Molecules

Different catalytic reactions of olefin and saturated hydrocarbon molecules, present in light naphtha require specific catalytic function(s). The relationship of these catalytic reaction(s) and surface electronic structure of different molybdenum oxidation states characterized by XPS-UPS techniques enabled us to identify the nature of the catalytic active phase in each system. Catalytic reactions of 1-pentene, cyclopentene, n-pentane, cyclopentane, 1-heptene, n-heptane and methyl cyclohexane molecules were studied on different MoO3, MoO2−x (OH) y and Mo(0) surfaces as a function of reaction temperatures. Distribution of products of Methylcyclohexane on Mo(0) phase as a function of reaction temperature.

Effect of phenol adsorption on HY zeolite for n-heptane cracking: Comparison with methylcyclohexane

To evaluate the possibility to partially replace the classical Fluid Catalytic Cracking (FCC) feedstocks by bio-oils derived from lignocellulosic biomass, the transformation of n-heptane was performed in the presence of small quantities of phenol over an HY zeolite, at 350 and 450 °C. The results were compared with those previously obtained with methylcyclohexane. Phenol leads to a further deactivation of the zeolite due to phenol adsorption on the Brönsted and Lewis acid sites along with coke molecules from the reactants transformation. For n-heptane, the additional activity decay due to phenol is observed since the beginning of the reaction, while for methylcyclohexane it was only detected after few minutes. The increase of temperature disfavours phenol adsorption but, contrarily to methylcyclohexane, it does not prevent the phenol deactivating effect for n-heptane. Differences in the reactivity of n-heptane and methylcyclohexane molecules and IR spectroscopy data allow concluding that phenol firstly interacts with the stronger protonic sites, being then retained on the weaker acid sites. Alkylation of phenol is a reaction occurring in a lower extent under the conditions tested, being responsible for the consumption of light olefins. Finally, the adsorption of phenol favours the formation of branched molecules. Hence, the alkanes cracking should be more affected than the naphthenes cracking in the co-processing of conventional FCC feedstocks with hydrotreated bio-oils rich in phenolic compounds, being the deactivating effect certainly dependent on the quantity of bio-oils incorporated.

Multiplicity of n‐heptane oxidation pathways catalyzed by cytochrome P450

Modeling, mutagenesis, and kinetic studies have demonstrated that the substrate-binding site of cytochrome P450 is composed of multiple interactive regions that are capable of simultaneously binding two or more xenobiotics. Substrate molecules can interact with each other after docking. Thus, substrates can compete for the activated oxygen-ferrous complex or alter the spatial orientation of other molecules. Cytochrome P450 is a unique enzyme that produces n-heptane metabolites of different oxidation states. Metabolism of n-heptane was investigated with rat liver microsomes and a reconstituted rat liver system. Ethanol, n-propanol, and n-butanol molecules interacted with the n-heptane molecule and resulted in cytochrome P450 spectral changes as well as alterations in the n-heptane metabolic profile. The observed modifications in the biotransformation of n-heptane indicated that there are three distinct pathways for oxidation of n-heptane to heptanols, heptanones, and one-side oxygen-oriented heptanediones.

Enhancement of Adsorption Inside Single-Walled Carbon Nanotubes: Li Doping Effect on n-Heptane van der Waals Bonding

Doping of opened single-walled carbon nanotubes (SWNTs) with metallic Li has been carried out under ultrahigh vacuum. Li atoms enter into the interior sites of the SWNTs by surface migration with an activation energy of 0.3 ± 0.04 eV. Density functional theory calculations indicate that the Li atoms ionize inside the SWNTs to produce Li+ ions. The Li-doped SWNTs exhibit an ∼10% enhancement of the van der Waals bonding for n-heptane molecules, a model for volatile organic compounds, within the interior due to polarization of the alkane molecules in the strong electrostatic field around the Li+ ions. The investigations reveal the utility of temperature-programmed desorption from SWNTs for understanding the energetics of confined molecule adsorption in pristine and doped SWNTs. Statistical mechanical simulations using a polarizable potential for alkanes give increases in isosteric heats of adsorption at high alkane loading that are in excellent agreement with experiments. Our simulations predict that the bin...

Heptane adsorption in silicalite-1: Molecular dynamics simulation

Molecular dynamics (MD) simulations have been used to study the adsorption process of n-heptane molecules in silicalite-1 at 300 K. MD simulated results were compared to experimental neutron diffraction (ND) and experimental self-diffusion coefficients. The analysis of MD data indicated a packing of the adsorbed molecules around 4 mol./u.c., which is not the consequence of an enthalpic effect but of an entropic effect. The role of the n-heptane chain flexibility ( cis– trans conformation) in relation with the silicalite-1 channel type (straight versus sinusoidal) was outlined and enabled to understand the mobility change arising at 4 mol./u.c., according to previous experimental results. The MD simulation also allowed to identify adsorption sites, three in the straight channels and three in the sinusoidal channels and to characterize their position, energy and occupation. Site position but only relative occupation data were in good agreement with neutron diffraction data. The assumption of a “commensurate freezing” to explain the step isotherm is discussed in the light of the MD simulation and ND refinement results.

Experimental and Simulation Study of n-Heptane Adsorption on Rutile

The adsorption of n-heptane on microcrystalline rutile has been studied experimentally by thermodynamic techniques (adsorption isotherms and microcalorimetry) over a wide range of coverage at 303 K and complemented by Grand Canonical Monte Carlo simulations. The differential heat of adsorption exhibited three descending segments corresponding to the adsorption of n-heptane on three types of surfaces. The mean molar adsorption entropy of n-heptane in the monolayer was less than the entropy of the bulk liquid by ca. −23 J/(mol K), thus revealing a hindered state of motion for the n-heptane molecules on the surface of rutile. Simulations of the adsorption of n-heptane were performed on the three most abundant crystallographic faces of rutile. The adsorption isotherm obtained from the combination of the isotherm for each face weighted by the respective abundance was found to be in good agreement with experimental data. A structural characterization of n-heptane near the surface was also conducted which indicated that the substrate strongly perturbed the distribution of the n-heptane conformations relative to the situation found for the gaseous phase. Adsorbed molecules are predominantly orientated with their long axes, with the zig-zag planes of their backbones parallel to the surface and preferentially aligned along the five-fold cus Ti4+ ions of the faces. Fewer gauche conformations were observed for molecules near the surface than was characteristic of the bulk phase.

Molecular Dynamics Studies of the Stability of Water/n-Heptane Interfaces with Adsorbed Naphthenic Acids

The mechanism by which naphthenic acids stabilize water/oil interfaces has received extensive attention because of its industrial relevancy. In this work, we employed a molecular dynamics simulation to study its molecular origin. Two models were adopted, wherein naphthenic acid coverage of water/n-heptane interfaces, both spherical and flat, was hypothesized, respectively. It was found that the coalescence of two water clusters is entirely attributed to the diffusional motion of the components involved which requires the initial departure of the naphthenic acid molecules from the interface so that a water bridge can form. The naphthenic acids not only act as a steric barrier but also reduce the mobility of the water and n-heptane molecules making the formation of the water bridge rather difficult. In fact, our results show that the coalescence of two water clusters fully covered by naphthenic acid molecules is a low-probability event even at evaluated temperatures. In addition, the results from the flat interface models suggest that the emulsion stability is weakly dependent on the molecular weight of the naphthenic acids utilized. Order parameter calculations reveal liquid-crystal-like ordering of naphthenic acids at the water/n-heptane interfaces. All these observations are consistent with the corresponding experimental observations. The present work also suggests that the mobility of naphthenic acids is considerably enhanced with more n-heptane molecules present outside a water droplet. However, in such a case, coalescence could not occur as the water clusters are far apart from each other.