The mechanism of ethylene pyrolysis at small conversions

Kinetic modelling is used in conjunction with measurements of product yields to develop a mechanism for the pyrolysis of ethylene at 896K and ethylene pressures ranging from approximately 3 to 78 kPa. An induction period was observed for all products except H₂, and was followed by a steady rate, which was of second-order for all products except 1,3-C₄H₆, the most abundant product. The mechanism quantitatively accounts for the yields of H₂, CH₄, C₂H₆, C₃H₆, 1-C₄H₈ and 1,3-C₄H₆. The reaction is initiated by disproportionation of C₂H₄ and the product 1,3-C₄H₆ results from decomposition of the C₄H₇ radical, formed by addition of C₂H₃ to C₂H₄. The other organic products that were measured are formed as a result of reactions involving the C₂H₅ radical. The hydrogen is produced by abstraction from C₂H₄ by atomic hydrogen and its rate is controlled by the reaction C₂H₅ → C₂H₄ + H which is nearly equilibrated. The main termination reaction is recombination of C₂H₅. The auto-acceleration which is evident particularly in the yields of H₂, CH₄, C₂ H₆, and C₃H₆ is accounted for by the decomposition of 1-C₄H₈. © 1996 John Wiley & Sons Inc.     

Photoinduced formation of hydrogen peroxide in aqueous solutions of adenine derivatives at 77 K

The amount of hydrogen peroxide in aqueous solutions of adenine (A), adenosine (Ado), cytidine (Cyt), and thymine (T) containing 0.1 M NaCl and irradiated with near-UV light at 77 K is determined. It is established by comparing the results to data obtained earlier that the amount of H₂O₂ detected in the defrosted samples following identical irradiation falls in the order Ado > adenosine-5′-diphosphate (ADP) > A >> Cyt. The formation of H₂O₂ was not detected for T. The formation of H₂O₂ in solutions of adenine derivatives was observed when the samples were irradiated with light having wavelengths in the ranges λ = 240–400 nm and 290–450 nm. The latter covers only the long wave absorption range of these compounds. It is shown that the change in the intensity of irradiation that strongly affected the intensity оf EPR signals of irradiated samples prior to defrosting affected the amount of detected H₂O₂ only slightly, and the effect was not unidirectional. The results from determining H₂O₂ in the samples of adenine derivatives are compared to estimates of the content of free peroxyl radicals, obtained by analyzing EPR spectra. Plausible mechanisms of the processes are discussed.     

Nonisothermal effects in the process of soot formation in ethylene pyrolysis behind shock waves

The nonisothermal nature of hydrocarbon pyrolysis explains the differences in the critical temperatures of soot formation in the experimental studies of these processes. When reaction heats are taken into account, the critical temperatures become close to 1600 K for all the systems studied. The estimated standard enthalpy of carbon atom formation in the composition of soot particles is δH_{f, z}. ≈ 11 ±6 kJ/mol. A kinetic model is proposed for soot formation in ethylene pyrolysis that describes the experimental data. The calculated temperature of soot particles may differ substantially depending on the choice of a model for energy exchange in collisions.     

Electron scavenging by solutes in ethylene glycol-water glasses at 77 K

The efficiencies of a number of solutes in reducing the concentration of trapped electrons in gamma-irradiated ethyleneglycol-water glasses at 77 K has been studied, inclusive of trap depth variation studies. A model, involving the polarising power of the solutes has been suggested to explain the relative efficiencies of solutes in reducing the yield of trapped electrons.     

Ammonium hydrogen tartronate at 240 and 20 K

The low-temperature structure determination of the title compound, alternatively called ammonium hydrogen hydroxy­propane­dioate, NH₄⁺·C₃H₃O₅⁻, has revealed that the H atom involved in a very short asymmetric O—H⋯O hydrogen bond [O⋯O = 2.448 (2) Å at 240 K and 2.4393 (10) Å at 20 K] is disordered.     

Sum frequency generation studies at poly(ethylene terephthalate)/silane interfaces: hydrogen bond formation and molecular conformation determination

To better understand the effects of interfacial molecular orientation on adhesion to plastics, the interfaces between poly(ethylene terephthalate) (PET) and different silane coupling agents were probed using sum frequency generation (SFG) vibrational spectroscopy. The polymer/air interface was dominated by the ester carbonyl, methylene, and phenyl groups. Upon contacting the PET film with the amino-functional silane 3-aminopropyltrimethoxysilane (ATMS), the ester carbonyl stretch shifted to a lower energy indicating the formation of hydrogen bonds between the polymer surface and the silane molecules. This shift was not observed when silanes that contained no hydrogen bond donors, such as (3-glycidoxypropyl)-trimethoxysilane and n-butyltrimethoxysilane, were placed into contact with the PET surface. Further evidence of silane ordering at the interface was observed as vibrational peaks attributed to the C-H stretching of the silane methoxy headgroups dominated the PET/silane spectra. It was determined that the conformation of the ATMS molecules at the interface was such that the amino endgroups were oriented toward the interface while the methoxy headgroups were directed toward the silane bulk.     

State of hydrogen in idealized carbon slitlike nanopores at 77 K

The purpose of this letter is to clarify recent findings and answer to the question: "What is the state of hydrogen in carbon slitlike pores at 77 K?" For this purpose, we determined the volumetric density of hydrogen in idealized carbon pores of molecular dimension at 77 K and pressure up to 1 MPa. We used quantum corrected grand canonical Monte Carlo simulation. We recognized the highest volumetric density of confined hydrogen (around 71% of hydrogen liquid at boiling point) for effective pore width 5.6 angstroms (H* = 3.04) in the considered pressure range. Our computational results are in agreement with the calculations performed by Wang and Johnson and Rzepka et al. In contrast, we did not observe the high volumetric density of hydrogen in slitlike carbon pores exceeding the density of hydrogen liquid at the boiling point as was reported by Jagiello and Thommes. Moreover, we obtained qualitative agreement between the simulation results and some experimental findings reported by Nijkamp.     

Kinetics of soot formation in tetrachloromethane pyrolysis

Kinetics of soot formation is studied in tetrachloromethane pyrolysis behind shock waves. The time dependences of macrokinetic characteristics of soot particle growth (the induction period, the soot yield, and the apparent rate constant of soot particle growth) are determined. Based on the experimental data, the quantitative model of soot formation is developed for tetrachloromethane pyrolysis behind shock waves. Special attention is paid to the thermal effects in CC1⁴ pyrolysis.     

Laser pyrolysis studies of OH reaction rates with several butenes at 1200 K

Using a laser pyrolysis/laser fluorescence method, we have measured the rate constants for OH reacting with 1-butene, t-2-butene, isobutene, and 2,3-dimethyl-2-butene near 1200 K. The butene rate constants are large, ranging from 2.0–3.7 × 10^?11 cm^?3 s^?1, and increase with the number of allylic hydrogens. Transition-state theory considerations indicate these allylic hydrogens are easily abstracted, in contrast to prior observations on propene.     

The formation of hydrogen peroxide in metallic reductors

The formation of hydrogen peroxide in various types of metallic reductors both in the presence and absence of oxygen has been studied. Only when oxygen is rigorously excluded is peroxide undetectable. The oxidimetric determination of iron is seriously affected by this peroxide when the test solution and reductor are open to atmospheric oxygen. Systems which are completely oxygen-free give satisfactory results.     

Adsorption of hydrogen on multiwalled carbon natotubes at 77 K

Adsorption of H₂ on multiwalled carbon nanotubes was measured at 77 K by a volumetric method. Adsorption and desorption isotherms are largely reversible. The adsorption capacity increased remarkably after receiving heat treatment at 400 °C and being pressed into pellets. The isotherms show typical feature of supercritical adsorption and were satisfactorily modeled by the model that applied for usual supercritical adsorption. The maximal adsorption capacity of hydrogen under the condition tested is less than 0.5 wt%.     

Adsorption of molecular hydrogen on aluminophosphate zeolites at 77 K

The H₂ sorption properties of the aluminophosphate zeolites AlPO-5, AlPO-31, AlPO-11, AlPO-36, and AlPO-8 at 77 K have been investigated. A series of H₂ adsorption isotherms has been obtained for cylindrical micropore channels in the aluminophosphate zeolites. The absolute values of the amount adsorbed α(P) for the mesoporous aluminophosphate materials and the effective density of adsorbed H₂ in the micropore space β*(P, d) have been determined. It has been demonstrated experimentally that the sorbate density depends on the size of the micropore channel of the zeolite d. Hydrogen sorption isotherms have been calculated from experimental isotherms. A procedure allowing β*(P, d) to be estimated for intermediate d values is presented.     

The thermal diffusion characteristics of hydrogen and alkali metal chlorides in aqueous-methanolic solutions at 298.15 K

The paper presents the results of experimental studies of thermoelectrochemical systems comprising silver chloride and quinhydrone electrodes in aqueous-methanolic solutions of hydrogen, lithium, potassium, and cesium chlorides. These results and literature data are used to calculate the standard entropies of transfer of electrolytes, Soret thermal diffusion coefficients, and entropies of moving ions at 298.15 K. The influence of the nature of electrolytes and mixed solvent composition on the characteristics of thermal diffusion transfer in the systems studied is discussed.     

Formation of N-butane in the pyrolysis of ethylene [1]

Yields of n-C₄H₁₀ have been measured from the flow pyrolysis of C₂H₄ at 897 (±7) K. From 77 to 720 Torr the order of n-C₄H₁₀ formation was found to be 2.0±0.3 The rate constant for the reaction, was estimated to be 2.4(±0.6)×10^–4l mol^–1s^–1.

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-C₄H₁₀ C₂H₄ 897 (±7) . -C₄H₁₀ 77 720 2.0±0.3. 2C₂H₄C₂H₃+C₂H₅ 2,4 (±0,6)×10^{–4 –1–1}.

     

Radical pair formation in dihalogenated benzene derivatives at 77K

Radical pair formation in dihalogenated benzene derivatives irradiated at 77 K has been revealed by the EPR detection of ΔM = 2 transitions. In some specimens, the hydrogen atom signal has been observed, suggesting the presence of the phenyl radical.