The formation of hydrogen in ethylene pyrolysis at 900 K

Ethylene at concentrations of 2.7 × 10^?3 to 1.0 × 10^?2 mol L^?1 has been pyrolyzed at 900 K in a flow system. The products ethane and hydrogen have been analyzed by gas chromatography. The results are consistent with a mechanism in which these products are initially formed as follows: Reaction [1] occurs only 1 to 2% as often as the addition reaction, The latter reaction is close to equilibrium. Taking the rate constant, k₄, and the equilibrium constant, K₂, from the literature and making small adjustments for minor processes, k₁ is found to be (9 ± 3) × 10⁷ L mol^?1 s^?1. Here the uncertainty is intended to encompass errors in the present work and in the literature parameters. A secondary source of hydrogen was also observed. Its dependence on ethylene concentration was consistent with formation from an intermediate with six carbon atoms, such as cyclohexene.     

The pyrolysis of neopentane at small extents of reaction

The pyrolysis of neopentane, at small extents of reaction, was studied by gas chromatography, in Pyrex reaction vessels between 450° and 530°C and in the initial pressure range 25–200 mm Hg. At initial time, this thermal decomposition can be essentially represented by a homogeneous long-chain radical mechanism. The rate constant of the unimolecular initiation process is approximately given by the expression The initial rate constant of the global reaction (order 3/2) is nearly equal to This reaction is strongly inhibited by propene or isobutene and self-inhibited by the isobutene formed; an interpretation of all these inhibition phenomena of the neopentane pyrolysis is proposed. Our observations and conclusions, which have been summarized in communications during 1968 and 1969, are compared to those of other authors, particularly to the recent ones of Purnell and colleagues [13] and of Taylor and colleagues [14], [15].     

The pyrolysis of dimethyl sulfide,kinetics and mechanism

The kinetics of the gas phase pyrolysis of dimethyl sulfide (DMS) was studied in a static system at 681–723 K by monitoring total pressure-time behavior. Analysis showed the pressure increase to follow DMS loss. The reaction follows two concurrent paths: with a slow, minor, secondary reaction: In a seasoned reactor the reaction follows a 3/2 order rate law with rate coefficient given by with θ = 2.303 RT in kcal/mol. A free radical mechanism is proposed to account for the data and a theoretical rate coefficient is derived from independent data: which agrees well with the experimental one over the range studied. The reaction is initiated by Me₂S → Me + MeS? and propagated by metathetical radical attack on Me₂S. C₂H₄ is formed by an isomerization reaction which may in part be due to a hot radical: Thermochemical data are listed, many from estimations, for both molecular and radical species of interest in the present system.     

A pyrolysis mechanism for ammonia

The mechanism of NH₃ pyrolysis was investigated over a wide range of conditions behind reflected shock waves. Quantitative time-history measurements of the species NH and NH₂ were made using narrow-linewidth laser absorption. These records were used to establish an improved model mechanism for ammonia pyrolysis. The risetime and peak concentrations of NH and NH₂ in this experimental database have also been summarized graphically. Rate coefficients for several reactions which influence the NH and NH₂ profiles were fitted in the temperature range 2200 K to 2800 K. The reaction and the corresponding best fit rate coefficients are as follows: with a rate coefficient of 4.0 × 10¹³ exp(?3650/RT) cm³ mol^?1 s^?1, with a rate coefficient of 1.5 × 10¹⁵T^?0.5 cm³ mol^?1 s^?1 and with a rate coefficient of 5.0 × 10¹³ exp(?10000/RT) cm³ mol^?1 s^?1. The uncertainty in rate coefficient magnitude in each case is estimated to be ±50%. The temperature dependences of these rate coefficients are based on previous estimates. The experimental data from four earlier measurements of the dissociation reaction were reanalyzed in light of recent data for the rate of NH₃ + H → NH₂1 + H₂, and an improved rate coefficient of 2.2 × 10¹⁶ exp(?93470/RT) cm³ mol^?1 s^?1 in the temperature range 1740 to 3300 K was obtained. The uncertainty in the rate coefficient magnitude is estimated to be ± 15%.     

Non quasi-stationary state pyrolysis. Kinetic parameters of neopentane pyrolysis mechanism

The study of neopentane pyrolysis during its induction period has allowed to determine, from the quantitative measurement of major primary products only, the rate constant of the limiting chain propagation step, as well as the kinetic parameters of initiation and termination steps.

---

, , , , .

     

O-乙酰基-吡喃木糖热解反应机理的理论研究

为了从微观上理解半纤维素热解过程及其主要产物的形成演变机理,采用密度泛函理论方法B3LYP/6-31G++(d,p),对O-乙酰基-吡喃木糖的热解反应机理进行了量子化学理论研究。在热解过程中,O-乙酰基-吡喃木糖中的O-乙酰基首先脱出,形成乙酸和中间体IM₁,该步反应能垒为269.4 kJ/mol。IM₁进一步发生开环反应形成IM₂,开环反应能垒较低,为181.8 kJ/mol。对中间体IM₂设计了四种可能的热解反应途径,对各种反应的反应物、产物、中间体和过渡态的结构进行了能量梯度全优化,计算了各热解反应途径的热力学和动力学参数。计算结果表明,反应路径(4)和反应路径(2)是O-乙酰基-吡喃木糖热解的主要反应通道,乙酸、乙醛、乙醇醛、丙酮、CO、CO₂、CH₄等小分子产物是热解的主要产物。这与相关实验结果分析是一致的。     

The mechanism of the high-pressure free radical polymerization of ethylene

The reaction path of the free radical polymerization of ethylene is usually considered identical to the polymerization mechanism of other vinyl monomers. Available experimental data on the polymerization of ethylene, however, hardly fitted the well-established path of free radical polymerization. Obviously the mechanism of ethylene polymerization is more complex and not well understood. One reason for this, in our opinion, is insufficient knowledge of the physicochemical state of ethylene under high pressure. A model that described the behavior of ethylene under compression has been proposed. According to the model, and increase in pressure causes the formation of various supermolecular forms of ethylene, each accompanied by transition of the second order. By proposing a stereochemical shape for each supermolecular form calculation of activation volumes for each of these transitions was made. Good agreement was obtained when calculated volumes of activation were compared with corresponding experimental values in the literature.     

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.

---

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

     

One-step multi-electron transfer mechanism of polynuclear copper complexes for molecular conversions

The oxidative coupling reaction of 2,6-dimethylphenol may result in either a desired polymeric substance (i.e. the polyphenylene ether, PPE) or the undesired “dimeric” species diphenoquinone, DPQ. The relative amounts of each product depend on the experimental conditions and the used catalytic system. Usually copper amine compounds are used as a catalyst for the oxidative coupling reactions. They have the advantage of easy access and produce high yields of high molecular PPE; however, other metal coordination compounds, like those of Mn, may also be used as catalysts. The present paper focuses on mechanistic studies with various copper (aliphatic and aromatic) amine compounds as catalysts. Owing to the steric constraints of the amine ligands, dinuclear Cu(II) compounds, with small bridging anionic ligands, are easily formed. Such species are believed to be the catalyst precursors. Upon addition of a base (1:1 on copper) and excess phenol, phenolate ligands coordinate as bridging ligands to copper. After a two-electron transfer reaction, the resulting phenoxonium ligand, which is a rather poor ligand, remains attached to the Cu(I), probably coordinating via its aromatic ring. Nucleophilic attack by a phenol to the phenoxonium ion at the 4-position is likey to be most important to the coupling reaction. In the beginning of the reaction the undesired side product DPQ is also formed via a C–C coupling reaction. With copper(II) compounds containing imidazole-type chelating ligands, good activity was obtained; in the case of pyrazole-based and bridging S-donor chelating ligands, that no or very weak activity was found. In a study of the mechanism of the propagation reaction the rate-determining reaction was thought to be probably a one-step, two-electron transfer, during which the two Cu(II) ions in the dinuclear complex oxidize the phenolate to phenoxonium. After the phenoxium ion is formed the bonding with the (then) Cu(I) species is weakened and the reactions with phenolic end groups can take place. The effect of the amine ligands appears to be both steric and electronic. With certain ligands the reoxidationof the reduced catalyst is not possible.     

The photo-induced pyrolysis of ethylene in a cw CO2 laser beam

Ethylene gas was irradiated using a 660 W/cm² cw CO₂ laser emitting at 10.6 μm. When the pressure increases, both the light absorption (α = 0.016 torr^?1 cm^?1) and the chemical decay rate reach a limiting value. Poor experimental agreement with Landau—Teller or SSH theory is observed when T changes, and the limiting reaction rate coincides with a limiting quantum yield, from which a corresponding activation energy of 192.4 kJ/mole is deduced. The qualitative mechanism of decomposition can be explained, assuming a heat cooling control of the output molecular energy deposited through the translational degrees of freedom, which is similar to the control by heat transport of the fluorescence slow decay rate.     

The kinetic and mechanism of the gas phase pyrolysis of 1,1,1,2-tetrachloropropane

The gas phase pyrolysis of 1,1,1,2-tetrachloropropane was studied in a static system and seasoned vessel over the temperature range of 393.0–452.8°C and pressure range of 27.5–118.5 torr. The reaction is homogeneous, unimolecular, follows a first-order rate law, and is not affected by the presence of the free radical inhibitor, toluene, or propene. The main dehydrochlorination products are 1,1,1-trichloro-2-propene and 1,1,2-trichloro-1-propene. The temperature dependence of the rate coefficient is given by the Arrhenius equation The Arrhenius parameters of this reaction are apparently low. Consequently, in estimating the transition state of A = 10^13.5, the following expression is obtained The partial rates and kinetic parameters for the parallel elimination products have been estimated and reported. The trichloromethyl substituent has been found to affect this elimination process through its electron withdrawing effect.     

The kinetics and mechanism of the pyrolysis elimination of methyl 4-bromocrotonate

The pyrolysis of methyl 4-bromocrotonate in the temperature range 300–340°C and pressure range 74–170 torr has been shown to be homogeneous, unimolecular, and to follow a first-order rate law. The reaction was carried out in a static system, seasoned with allyl bromide, and in the presence of the radical chain exhibitor toluene. The rate coefficients are represented by the Arrhenius expression: log k₁(s^?1) = (13.30 ± 0.66) ? (185.2 ± 7.5) kJ mol^?1 (2.303RT)^?1. The carbomethoxy group appears to provide anchimeric assistance in the process of dehydrobromination and lactone products formation. The partial rates for the parallel reaction have been estimated, reported, and discussed. The pyrolysis elimination is explained in terms of an intimate ion pair-type of mechanism.     

Toward a comprehensive mechanism for methanol pyrolysis

A single kinetic mechanism for methanol pyrolysis is tested against multiple sets of experimental data for the first time. Data are considered from static, flow, and shock tube reactors, covering temperatures of 973 to 2000 K and pressures of 0.3 to 1 atmosphere. The model results are highly sensitive to the rates of unimolecular fuel decomposition and of various chain termination reactions that remove CH₂OH and H radicals, as well as to experimental temperature uncertainties. The secondary fuel decomposition reaction CH₃OH = CH₂OH + H, which has previously been included only in mechanisms for high temperature conditions, is found to have a significant effect at low temperatures as well, through radical recombination. The reaction CH₃O + C = CH₃ + CO₂, rather than CH₃OH + H = CH₃ + H₂O, is found to be the dominant source of CH₃ at low temperatures. The reverse of CH₃ + OH = CH₂OH + H is important to CH₃ production at high temperatures.     

Kinetics and mechanism of the pyrolysis of pentachloroethane

The rate of the inhibited pyrolysis of pentachloroethane was studiedover the temperature range of 820 to 865°K using the toluene-carrier technique in a stirred-flow reactor. The pyrolysis rate was found to be first order in reactant, and the rate constant is described by k=10^11.6±0.7 exp [(?48,200±2600)/RT] sec^?1. An increase by a factor of 6.6 in the surface/volume of the reactor had a negligible effect on the rate. This observation, in addition to a reevaluation of earlier kinetic data for the pyrolysis of pentachloroethane, lead to the following conclusions concerning the pyrolysis mechanism. The initiation and termination as well as the propagation reactions were homogeneous, the termination involved both Cl and C₂Cl₅ radicals (crosstermination), and autocatalysis was caused by interaction between chlorine and pentachloroethane rather than by dissociation of molecular chlorine.