Kinetic study-pyrolyses

Several new facets of the chemical and physical behavior of mutagens isolated from food and pyrolysates have been noted recently. Trp-P-1, Trp-P-2, and Glu-P-1 are rapidly deaminated upon incubation with nitrite at acid pH (54). At pH 1.6, in 50 yM nitrite, the half lifetime of Trp-P-1 and Trp-P-2 is approximately 100 min, but less than 5 min for Glu-P-1. AdC is also deaminated in 1 mM sodium nitrite at pH less than four with the difference that longer incubation, for 1.5 h, leads to the formation of a directly mutagenic nitroso derivative (55). These reaction conditions approach those in the stomach (pH 1-2, 0-10 yM nitrite), but careful kinetic studies in vivo will be required... 

Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy has been proven to be an excellent means of characterizing coals and related materials. This report is devoted to the evaluation of the technique as a method for situ monitoring of the chemical structural changes wrought in reactions of coal with fluid phases. This technique does not require a supporting medium (matrix) which can contain chemical artifacts which inherently serve as a barrier for access to the solid coal. The rapid response of the Fourier transform infrared technique is further beneficial for kinetic studies related to combustion, liquefaction, gasification, pyrolyses, etc. Experimental equipment and techniques are described for studies over wide ranges of pressure (10 5 Pa to ca 1.5 x 10 kPa) and temperature (298 K to 800 K). 

Sakai, T., et al. A Kinetic Study on the Formation of Aromatics During Pyrolysis of Petroleum Hydrocarbons, in Albright, L. F. und Crynes, B. L. Industrial and Laboratory Pyrolyses. ACS Symposion Series 32, Washington 1976, p. 152-177. 

An innovative micro-reaction system, based on the Curie Point Pyrolyser, has been developed at the University of Calgary and successfully employed for kinetic studies dealing with ultrapyrolysis of various hydrocarbons, including propane, n-hexadecane and heavy oils (Rastogi et al., 1988a Fairburn et al., 1990 Tan and Behie, 1991). 

Thermal analysis and kinetic studies have demonstrated that oxidative reactions are responsible for acceleration in the rates of weight loss and depolymerisation of cellulose pyrolysed in air at temperatures below 300 °C [7], as well visible in the TG and dTG curves plotted in Figure 14.5. 

Pyrolysis where the sample is repeatedly pyrolysed for a short time under identical conditions (kinetic studies). 

It is appropriate to start with BaN6 since this compound has been studied particularly intensively and has been regarded as a model in the development of the theory of kinetics of decompositions of solids. The sigmoid a—time curves for BaN6 pyrolyses, Fig. 15, are typical examples of solid state autocatalytic behaviour. 

Pyrolyses of formates, oxalates and mellitates yield CO and C02 (H2, H20 etc.) as the predominant volatile products and metal or oxide as residue. It is sometimes possible to predict the initial compositions from thermodynamic considerations [94], though secondary reactions, perhaps catalyzed by the solids present, may result in a final product mixture that is very different. The complex mixtures of products (hydrocarbons, aldehydes, ketones, acids and acid anhydrides) given [1109] by reactants containing larger organic groupings makes the collection of meaningful kinetic data more difficult, and this is one reason why there are relatively few rate studies available for the decompositions of these substances. 

Hydrocarbons and hydrogen halides are omitted since they will be dealt with elsewhere.) The chemical properties of most of these hydrides are rather well known, but this cannot be asserted for their decomposition kinetics. Some of them are very stable (H20, HF, NH3) while others decompose easily at room temperature (TeH2, PbH4). A study of the homogeneous decomposition has only been undertaken for those elements inside the frame in the Table. The pyrolyses of the others have either been found to proceed heterogeneously or the kinetics is unknown. 

Since we are primarily concerned with kinetics, we do not intend to discuss all the proposed mechanisms or the numerous boron hydrides formed during the pyrolyses. The literature available on this field up to 1964 has been reviewed by Adams81 and by Lipscomb85. Rapid progress is being made in studies of the kinetics of boron hydride decomposition so that this discussion must be regarded as preliminary. 

The kinetics associated with the reactions shown in Figure 7 are summarized in Table n. Detailed mechanistic studies on the pyrolysis of alkylaromatics (12,13,15), alkylnaphthenes (14) and alkyltetralins (14) have allowed for the formulation of the Arrhenius parameters and stoichiometric coefficients shown. The kinetics for paraffin and olefin pyrolyses were extracted from the abundant literature data (16-18). Finally, the issue of kinetic interactions have been both theoretically and experimentally addressed (11,19). These interactions in general cause the reaction of the mixture to be different then the linear combination of the pure component rates. 

Heterogeneous or surface effects have been found to complicate the interpretation of kinetic experiments, which lead to erroneous Arrhenius parameters. However, with special precautions involving the use of seasoned vessels and the presence of a free-radical suppressor, the errors are minimized. Consequently, the present chapter will cover mostly homogeneous gas-phase processes. Studies on chemical activation, the use of catalysts, the bimolecular gas phase and heterogeneous reactions are not included. As an attempt to describe important pyrolyses data from 1972 to 1992, this review does not pretend to offer a complete coverage of the literature. 

In a study of the effect of alkyl and polar groups on the gas-phase pyrolyses of a-substi-tuted ethyl chlorides70, pinacolyl chloride was found to produce dimethylbutenes by kinetic control. The main alkene product, i.e. 3,3-dimethyl-1-butene, was formed through the normal four membered transition state for HC1 elimination. However, the formation of... 

To begin the exploration of actual reaction pathways in complex pyrolyses of aromatic substances, we have carried out a detailed experimental and theoretical analysis of the liquid-phase pyrolysis of bibenzyl. This pyrolysis system has been studied by others (44,45,46), and the general kinetic features of this reaction system are now rather well agreed on. Complete details of this work will appear elsewhere (38a) and a few implications of this work of particular relevance to coal reactions will be discussed here. 

The pyrolyses of a number of alkyl substituted cyclobutanes have been studied some kinetic parameters are given in Table 5. Pataracchia and Walters have investigated the methylcyclobutane pyrolyses at pressures down to 0.003 torr, and found the usual fall-off in the first-order rate coefficient on the basis of rrk theory the results could be fitted using s = 23, as compared with 18 for cyclobutane itself. 

The kinetics of a number of other hydrocarbon pyrolyses have been studied some recent references are acetylene, Kistiakowsky et al. benzene, Hou and Palmer toluene, Takeuchi et al. 1-butene, Trotman-Dickenson et al. 2-methyl-1-pentene, Taniewski 4-methyl-1-pentene, Taniewski . 

All of the theories that have just been mentioned were put forward at a time when few experimental details had been established for pyrolyses in the gas phase. During the past few years powerful techniques, particularly those of gas-liquid chromatography and mass spectrometry, have been developed, so that it is now possible to study the kinetics of formation of the minor products. 

Some such studies on the ethane pyrolysis in the presence of nitric oxide have been made by Pratt > who has been particularly concerned with the kinetics of formation of nitrogen, hydrogen, methane and butane. Work has recently been carried out at the University of Ottawa on the pyrolyses of ethane (Esser and Laidler ) and of acetaldehyde (Schuchmann and Laidler ). The technique of gas-liquid chromatography has been used for the most part to analyze the reaction... 

A number of kinetic isotope studies have been reported for gas-phase eliminations. Isopropyl bromide-dg decomposes more slowly than isopropyl bromide and the intramolecular isotope effects (A ,c2D4hx-c2D4)/ (C2D4hx-c2D.iH)) have been recorded for pyrolyses of ethyl acetate (2.0), chloride (2.20), and bromide (2.10) at 500°C. At the elevated reaction temperatures, these values correspond to the maximum predicted for complete loss of the C-H stretching vibration and they have been interpreted as indicating considerable weakening of the C-H bond in the transition state. Whether this is a homolytic or hetero-lytic bond fission, it is remarkably insensitive to beta substituent effects on rate. The intramolecular isotope effects in these cases could reflect predominantly a secondary isotope effect rather than the intended primary effect and dissection into an intermolecular and secondary isotope effect would prove more fruitful. (Section 2.2.1.)... 

This paper aims at summarizing our researches on the pyrolyses, at about 500 C, of four other alkanes -ethane (6a), isobutane (6b), n-butane (6c) and isopentane (6d)- in the presence of small oxygen concentrations (0.01 up to several %) and at initial pressures between 10 and 100 mm Hg. In an attempt to reach the chemical and kinetic features of the reactions without interference arising from the products, these reactions were studied at low percentage of conversion. Some preliminary results obtained in these investigations have already been succinctly presented (5)... 

Polyisoprene has also been pyrolysed in an inert atmosphere and here the main products are isoprene and l-methyl-4-isoprenylcyclohexene. The latter compound can disproportionate to l-methyl-4-isopropyIbenzene and methyl-l-iso-propylcyclohexenes and this reaction is catalysed by Ziegler-Natta catalyst residues or by carbon black. The dominant initiation process is j9-chain scission with the formation of two allylic radicals. The kinetics of thermal decomposition have been studied for cis- and rraiw-1,4-polyisoprene and the copolymer of isoprene with 4-isopropyl-methyl styrene and also for isoprene polymers containing 4-CjH4—Z—4-C(H4— and —CjH4—Z—C5H4N=N— units, where Z may be O, CHj, SOj or a single bond. ... 

The microreactor system proposed at the University of Calgary has been subsequently used to study the ultrapyrolysis kinetics of n-hexadecane, a gas oil model compound, n-hexadecane (about 12 fig per sample) has been pyrolysed at temperatures ranging from 576 to 842°C for reaction times of 100 to 3200 ms (including the temperature rise time TRT) and pressures in the range of 1 to 2 atm. 

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