High-temperature reactions of OH radicals with benzene and toluene

The rate constants for the reactions of OH radicals with benzene and toluene have been measured directly by a shock tube/pulsed laser-induced fluorescence imaging method at high temperatures. The OH radicals were generated by the thermal decomposition of nitric acid or tert-butyl hydroperoxide. The derived Arrhenius expressions for the rate constants were k(OH + benzene) = 8.0 x 10(-11) exp(-26.6 kJ mol(-1)/RT) [908-1736 K] and k(OH + toluene) = 8.9 x 10(-11) exp(-19.7 kJ mol(-1)/RT) [919-1481 K] in the units of cubic centimeters per molecule per second. Transition-state theory (TST) calculations based on quantum chemically predicted energetics confirmed the dominance of the H-atom abstraction channel for OH + benzene and the methyl-H abstraction channel for OH + toluene in the experimental temperature range. The TST calculation indicated that the anharmonicity of the C-H-O bending vibrations of the transition states is essential to reproduce the observed rate constants. Possible implications to the other analogous H-transfer reactions were discussed.     

Adsorption of benzene, toluene, and xylenes on monolithic carbon aerogels from dry air flows

Monolithic carbon aerogels were obtained by carbonization of organic aerogels prepared by polymerization of resorcinol and formaldehyde under different conditions. Some carbon aerogels obtained were further CO2-activated. Samples were characterized by gas adsorption, scanning electron microscopy, high-resolution transmission electron microscopy, and mechanical tests. Benzene, toluene and xylenes were adsorbed from dry air by using carbon bed columns, obtaining the breakthrough curves. There was no correlation between the amount adsorbed at the breakthrough point and the volume of micropores narrower than 0.7 nm. Conversely, a good linear relationship between the amount adsorbed at the breakthrough point and the total micropore volume up to a mean micropore width of around 1.05 nm was found. In addition, the height of the mass transfer zone decreased with the mean width of the total micropores up to a value of around 1.05-1.10 nm. One of the best adsorbents obtained showed the lowest height of the mass transfer zone and one of the highest amounts adsorbed at the breakthrough point, either per mass or volume unit. However, it had a lower elastic modulus and compressive strength than other monolithic carbon aerogels, although its compressive strength (3 MPa) was still high enough to use it in carbon bed columns. The sample with the best mechanical properties was a poorer adsorbent. Regeneration of the exhausted adsorbents allowed the recovery of the hydrocarbons adsorbed without any appreciable loss of adsorption capacity of the carbon bed.     

Adsorption of benzene,toluene, ethylbenzene and p-xylene by NaOCl-oxidized carbon nanotubes

Multiwalled carbon nanotubes (CNTs) were oxidized by sodium hypochlorite (NaOCl) solution and were employed as adsorbents to study their characterizations and adsorption performance of benzene, toluene, ethylbenzene and p-xylene (abbreviated as BTEX) in an aqueous solution. The physicochemical properties of CNTs such as purity, structure and surface nature were greatly improved after oxidation, which significantly enhanced BTEX adsorption capacity. The adsorption capacity of CNT(NaOCl) increased with contact time and initial adsorbate concentration, but changed insignificantly with solution ionic strength and pH. A comparative study on the BTEX adsorption revealed that the CNT(NaOCl) had better BTEX adsorption as compared to CNTs and granular activated carbon. This suggests that the CNT(NaOCl) are efficient BTEX adsorbents and that they possess good potential for BTEX removal in wastewater treatment.     

A computational study of surface tension determination of dense fluids as benzene,toluene, methanol,ammonia, ethylene,and carbon monoxide

In this study, the parameters of linear isotherm regularity, which called LIR equation state used to compute the surface tension of some dense fluids as benzene, toluene, methanol, ammonia, ethylene, and carbon monoxide. An expression has derived for radial distribution function (RDF) at constant temperature, g (σ), for a real fluid by the use of LIR. This expression, which is related to intermolecular interaction, can be used to describe the temperature–density dependency of RDF at constant temperature, g (σ, ρ, T). In addition, we derive an expression for surface tension of dense fluids (CO, C₆H₆, C₆H₅CH₃, CH₃OH, NH₃, and C₂H₄) using the LIR and g (σ, ρ, T). Unlike previous models, it has shown that, surface tension can obtain without employing ΔH and ΔS. Only P-V-T experimental data have been used to calculate the surface tension. Comparison of the calculated values of surface tension by LIR with the values obtained experimentally show this method is not precise. This problem has led us to try to obtain the expression for surface tension using the extended parameters A, B (A and B are the temperature-dependent parameters which noticeably are depended on attraction and repulsion). The obtained result shows that the accuracy of this method is very high and quite admissible.     

Accurate prediction for electron affinities of the radicals derived from the halide benzene

The molecular structures and electron affinities of eight radicals derived from the halide benzene by removing a hydrogen atom have been determined using seven hybrid Hartree-Fock/density-functional methods. The basis set used in this work is of double-zeta plus polarization quality with additional diffuse s- and p-type functions, denoted as DZP++. These methods have been carefully calibrated [J. C. Rienstra-Kiracofe, G. S. Tschumper, H. F. Schaefer, S. Nandi, and G. B. Ellison, Chem. Rev. (Washington, D. C.) 102, 231 (2002)]. The geometries are fully optimized with each density-functional theory method and discussed, respectively. The three different types of the neutral-anion energy separations reported in this work are the adiabatic electron affinity, the vertical electron affinity, and the vertical detachment energy. The most reliable adiabatic electron affinities (with zero-point vibrational energy correction), obtained at the DZP++ B3LYP level of theory, are 1.74 eV (o-C6H4F), 1.39 eV (m-C6H4F), 1.34 eV (p-C6H4F), 1.78 eV (o-C6H4Cl), 1.53 eV (m-C6H4Cl), 1.45 eV (p-C6H4Cl), 2.06 eV (o-C6H3F2), and 2.04 eV (p-C6H3F2), respectively. Compared with the experimental values, the average absolute error of the B3LYP method is 0.03 eV. The BLYP, BP86, and BPW91 functionals also gave excellent predictions, with average absolute errors of 0.05, 0.08, and 0.08 eV, respectively.     

Vapor-liquid equilibria for binary mixtures of carbon dioxide with benzene,toluene and p-xylene

Vapor-liquid equilibrium data for carbon dioxide - benzene, carbon dioxide - toluene, and carbon dioxide - p-xylene were measured for pressures up to 6.5 MPa, and at temperatures of 353 K, 373 K, and 393 K. The solubility of benzene in the dense carbon dioxide vapor phase is higher than that of either toluene or p-xylene. In the liquid phase, carbon dioxide is more soluble in p-xylene than in toluene or benzene. The experimental data obtained were compared with calculations from three correlations: the Peng-Robinson equation, the UNIFAC activity coefficient correlation, and the Perturbed-Anisotropic- Chain Theory (PACT). All three correlations predict phase compositions in good agreement with the experimental data.     

Computational study of radicals derived from hydroxyurea and its methylated analogues

Structural and electronic properties and chemical fate of free radicals generated from hydroxyurea (HU) and its methylated analogues N-methylhydroxyurea (NMHU) and O-methylhydroxyurea (OMHU) are of utmost importance for their biological and pharmacological effects. In this work the cis/trans conformational processes, tautomerizations, and intramolecular hydrogen and methyl migrations in hydroxyurea-derived radicals have been considered. Potential energy profiles for these reactions have been calculated using two DFT functionals (BP86 and B3LYP) and two composite models (G3(MP2)RAD and G3B3). Solvation effects have been included both implicitly (CPCM) and explicitly. It has been shown that calculated energy barriers for free radical rearrangements are significantly reduced when a single water molecule is included in calculations. In the case of HU-derived open-shell species, a number of oxygen-, nitrogen-, and carbon-centered radicals have been located, but only the O-centered radicals (e1 and z1) fit to experimental isomeric hyperfine coupling constants (hfccs) from EPR spectra. The reduction of NMHU and OMHU produces O-centered and N-centered radicals, respectively, with the former being more stable by ca. 60 kJ mol(-1). The NMHU-derived radical e4 undergoes rearrangements, which can result in formation of several conceivable products. The calculated hfccs have been successfully used to interpret the experimental EPR spectra of the most probable rearranged product 10. Reduction potentials of hydroxyureas, radical stabilization energy (RSE) and bond disocciation energy (BDE) values have been calculated to compare stabilities and reactivities of different subclasses of free radicals. It has been concluded, in agreement with experiment, that reductions of biologically relevant tyrosyl radicals by HU and NMHU are thermochemically favorable processes, and that the order of reactivity of hydroxyureas follows the experimentally observed trend NMHU > HU > OMHU.     

Electrochemical bromination of benzene,toluene and xylene in acetonitrile

A method for the preparation of bromo derivative compounds of aromatics is proposed and the electrodic process involved in the bromination on the third wave of the system Br^?/aromatic substrate in acetonitrile is discussed.     

Trace analysis of benzene,toluene, ethylbenzene and xylene isomers in environmental samples by low-pressure gas chromatography-ion trap mass spectrometry

A rapid determination of benzene, toluene, ethylbenzene and the three xylene isomers (BTEX), including a nearly baseline separation of the xylene isomers in environmental samples within 1 min has been carried out using low-pressure gas chromatography-ion trap mass spectrometry (LP-GC-IT-MS). In order to evaluate the different parameters which may influence the performance of LP-GC-IT-MS, different column and mass spectral parameters were varied. Comparing LP-GC-IT-MS with the conventional equivalent, we obtained excellent detection limits as well as a good RSD of 8-13% in ition to a much shorter analysis time. In order to evaluate LP-GC-IT-MS for use in environmental samples, we determined BTEX in air.     

Electrodecomposition in subcritical water using o-xylene as a model for benzene, toluene, ethylbenzene, and xylene pollutants

The possibility of the combination of electrolysis and subcritical water as a novel electrolyte was investigated. A stainless steel reactor was used as an undivided electrochemical cell containing platinum as the anode and a stainless steel reactor as the cathode. At first, the effect of temperature on the electrolysis current as the main parameter was studied in a cell containing only pure water and a supporting electrolyte. It was realized that the electrolysis current (and, consequently, the electrolysis efficiency) increased linearly with temperature because of the change in viscosity and other physicochemical properties of subcritical water. As a result, at 553 K the electrolysis efficiency was over 14-fold higher than that under ambient conditions. The possibility of the applicability of the above combined techniques for the decomposition of o-xylene was also followed as a model for benzene, toluene, ethylbenzene, and xylene (BTEX) compounds. The effect of experimental conditions such as the electrolysis duration, the electrolysis voltage, and the temperature of subcritical water was investigated. Several decomposed products were identified. o-Xylene was directly electro-oxidized to 2-methylbenzyl alcohol and consecutively to the other oxidation products. Also, hydroxide ions were oxidized to oxygen molecules, where hydrogen was generated on the cathodic surface. The final oxidation product of the electro-oxidation reaction was identified as carbon dioxide. The results indicate that more than 95% of o-xylene can be decomposed under optimum conditions.     

有机溶液中C60与对叔丁基杯[8]芳烃间的包合作用

在本工作中, 我们测量了苯、甲苯及四氯化碳中: C60和对叔丁基杯[8]芳烃及共混合液的紫外可见吸收光谱, C60和对叔丁基杯[8]芳烃及其包合物的饱合溶解度, 及包合物的浓度积;推算了上述有机溶液中C60与对叔丁基杯[8]芳烃间包合反应的平衡常数及液相中真正以包合形式存在的包合物的溶解度; 讨论了上述溶剂中包合物的存在形式、沉淀机理及溶剂效应。     

Pronounced non-Arrhenius behaviour of hydrogen-abstractions from toluene and derivatives by phthalimide-N-oxyl radicals: a theoretical study

Abstraction of hydrogen atoms by pthalimide-N-oxyl radicals is an important step in the N-hydroxyphthalimide catalyzed autoxidation of hydrocarbons. In this contribution, the temperature dependency of this reaction is evaluated by a detailed transition state theory based kinetic analysis for the case of toluene. Tunneling was found to play a very important role, enhancing the rate constant by a factor of 20 at room temperature. As a result, tunneling, in combination with the existence of two distinct rotamers of the transition state, causes a pronounced temperature dependency of the pre-exponential frequency factor, and, as a consequence, marked curvature of the Arrhenius plot. This explains why earlier experimental studies over a limited temperature range around 300 K found formal Arrhenius activation energies and pre-factors that are 4 kcal mol(-1) and three orders of magnitude smaller than the actual energy barrier and the corresponding frequency factor, respectively. Also as a consequence of tunneling, substitution of a deuterium atom for a hydrogen atom causes a large decrease in the rate constant, in agreement with the measured kinetic isotope effects. The present theoretical analysis, complementary to the experimental rate coefficient data, allows for a reliable prediction of the rate coefficient at higher temperatures, relevant for actual autoxidation processes.     

Diffusion-based calibration for solid-phase microextraction of benzene, toluene, ethylbenzene, p-xylene and chlorobenzenes from aqueous samples

Short-term solid-phase microextraction (SPME) was performed to test a recently proposed semi-empirical model for the prediction of concentrations of analyte in water samples from the fibre-extracted mass without further calibration. The mass uptake rates obtained for benzene, toluene, ethylbenzene and p-xylene (BTEX) differ considerably from the before published, showing that interfibre comparability is a serious issue. The relative prediction errors are between -55% for benzene and +82% for p-dichlorobenzene under optimal conditions, i.e. they are by an order of magnitude higher than originally published. A sensitivity analysis shows the dominant influence of the estimated thickness of the diffusional boundary layer around the fibre on the concentration predicted. Empirical modification of the model equation for this parameter yields satisfactory results under the conditions tested for both BTEX and the selected chlorobenzenes.     

Steam reforming of toluene as model of tar compound over Mo catalysts derived from hydrotalcites

This study evaluated the catalytic activity of Mo catalysts derived from hydrotalcite-like compounds for steam reforming of toluene as a model compound for tar. The catalysts with 1.5, 3 and 4.5 Mo loadings (wt%), denoted as Mo_1.5MgAl, Mo₃MgAl and Mo_4.5MgAl respectively, were prepared by coprecipitation and characterized by BET, XRD, SEM, TEM, FT-IR and UV–VIS. The results showed that toluene conversion increased with increasing molybdenum content. The hydrogen amount depended on two factors: the presence of molybdate species on the surface and the presence of aluminum cations in tetrahedral sites (Mo₃MgAl), with molybdenum influence being more pronounced. The H₂/CO ratio decreased at increasing temperature while, the H₂/CO₂ ratio increased proportionally with temperature. Mo_1.5MgAl catalyst was more selective for CO₂ and H₂, while, Mo₃MgAl and Mo_4.5MgAl were more selective for CO and H₂.     

Energy transfer from cis- and trans-decalin to benzene and toluene in their irradiated mixtures

The fluorescence from β^- particle irradiation of cis- and trans-decalin containing benzene or toluene has been studied as a function of aromatic concentration from ⋍ 0.002 to 0.1 M and over a spectral range that encompasses both the solvent and aromatic fluorescence. By comparisons with the fluorescence obtained using sub-ionization excitation of the decalin, the effect of benzene and toluene to intrude into the geminate ion-pair decay process has been extracted and rate constants for their scavenging action obtained via fitting to the standard diffusion model. The rate constants are compared to those reported from microwave conductivity studies on the “escaped” mobile hole in these liquids. For the reaction between trans-decalin⁺ + toluene, the rates are in good agreement. However, for the reactions of either cis- or trans-decalin⁺ with benzene, the rate constants extracted from the fluorescence analysis are about an order of magnitude larger. The discrepancies suggest the existence of differences in the internal energies and structures of the decalin positive ions when observed on the very short time scale of geminate recombination (probed in the fluorescence measurements) and that which is observed on the much longer time scales that are probed in the microwave experiments.An analysis of the development of aromatic fluorescence permits extraction of the fraction of aromatic fluorescence that derives from ionic recombination (as opposed to energy transfer) and the averaged efficiency of this recombination. In all of the systems studied here the ionic fraction remains high (i.e. >;20%) even at millimolar concentrations of the aromatic.     

Synthesis and biological activity of Schiff bases derived from dehydroabietylamine and benzaldehyde derivatives

Previously unknown Schiff bases were prepared from dehydroabietylamine and substituted benzaldehydes. Their physicochemical properties were determined, and the bactericidal activity toward some microorganisms was evaluated.     

Formation of ring-retaining products from the OH radical-initiated reactions of benzene and toluene

The aromatic ring-retaining products formed from the gas–phase reactions of the OH radical with benzene and toluene, in the presence of NO_x, have been identified and their formation yields determined. These products, and their formation yields, are as follows: from benzene – phenol, 0.236 ± 0.044; nitrobenzene, {(0.0336 ± 0.0078) + (3.07 ± 0.92) × 10^?16[NO₂]}; from toluene – benzaldehyde, 0.0645 ± 0.0080; benzyl nitrate, 0.0084 ± 0.0017; o?cresol, 0.204 ± 0.027; m? + p?cresol, 0.048 ± 0.009; m-nitrotoluene, {(0.0135 ± 0.0029) + (1.90 ± 0.25) × 10^?16[NO₂]}, where the NO₂ concentration is in molecule cm^?3 units. The formation yields of o- and p-nitrotoluene from toluene were ca. 0.07 and 0.35 that of m-nitrotoluene, respectively. The observations that the nitro-aromatic yields do not extrapolate to zero as the NO₂ concentration approaches zero are not consistent with current chemical mechanisms for these OH radical-initiated reactions, and suggest that under the experimental conditions employed in this study the hydroxycyclohexadienyl radicals formed from OH radical addition to the aromatic ring react with NO₂ rather than with O₂. However, these data concerning the nitroaromatic yields are consistent with our previous conclusions that many of the nitrated polycyclic aromatic hydrocarbons present in ambient air are formed, at least in part, in the atmosphere from OH radical reactions.