Mechanism pyrolysis

The second type of pyrolysis mechanism is completely different and involves free radicals. Initiation occurs by pyrolytic homolytic cleavage. The remaining steps may vary, and a few are shown... 

Matrix IR spectra of various silenes are important analytical features and allow detection of these intermediates in very complex reaction mixtures. Thus, the vibrational frequencies of Me2Si=CH2 were used in the study of the pyrolysis mechanism of allyltrimethylsilane [120] (Mal tsev et al., 1983). It was found that two pathways occur simultaneously for this reaction (Scheme 6). On the one hand, thermal destruction of the silane [120] results in formation of propylene and silene [117] (retroene reaction) on the other hand, homolytic cleavage of the Si—C bond leads to the generation of free allyl and trimethylsilyl radicals. While both the silene [117] and allyl radical [115] were stabilized and detected in the argon matrix, the radical SiMc3 was unstable under the pyrolysis conditions and decomposed to form low-molecular products. 

Influence of Metal Ions on Oxygen Chemisorption and Ignition of Chars. We have carried out extensive studies of the influence of metal ions in wood on pyrolysis mechanisms (5.6) and this approach has now been extended to oxygen chemisorption of the chars. The metal ions occur in wood predominantly as the counterions of the uronic acid components of the hemicelluloses (12). We have shown that they can be almost completely removed by very mild acid treatment without any other major change in the chemistry of the wood. Table II shows that the major metal ions in cottonwood are Ca, K and Mg. The acid-washing process removed 98X of the metal ions in... 

To discriminate between the combustion-reforming mechanism and the pyrolysis mechanism, one must clarify whether CO or C02 is the primary product (or whether both are). Typically, to discriminate between primary and secondary... 

Although numerous authors (17,129,130,134-137) suggested that the dissociation of methane constitutes the first step in the methane oxidation by the pyrolysis mechanism, it is important to provide direct evidence for the formation... 

The thermal decomposition and photolysis of this alkyl have been studied by Buchanan and Creutzberg112. The pyrolysis mechanism is not fully understood. The overall process is first-order and is unaffected by an 8.5-fold increase in surface-to-volume ratio. Based on measurements of pressure increase, the reaction exhibits an induction period ranging from 2-3 minutes at 513 °C to 40 minutes at 466 °C. Short chains are apparently involved. A polymer initially of empirical formula (BCH2) but slowly losing hydrogen to form (BCH) is deposited on the surface. The mechanism probably involves the reactions... 

An ab initio study of the unimolecular pyrolysis mechanisms of monothioformic acid (363) has yielded activation energies for dehydrogenation and dehydrosulfidation. ... 

A CH4 pyrolysis mechanism appears to be consistent with our observation that preheating improves partial oxidation selectivity. First, higher feed temperatures increase the adiabatic surface temperature and consequently decrease the surface coverage of O adatoms, thus decreasing reactions lOa-d. Second, high surface temperatures also increase the rate of H atom recombination and desorption of H2, reaction 9b. Third, methane adsorption on Pt and Rh is known to be an activated process. From molecular beam experiments which examined methane chemisorption on Pt and Rh (79-27), it is known that CH4 must overcome an activation energy barrier for chemisorption to occur. Thus, the rate of reaction 9a is accelerated exponentially by hi er temperatures, which is consistent with the data in Figure 1. 

This unexpected result may be related to the increase in TOC on fraction G3 and may be further evidence of the polymerization phenomenon discussed earlier. However, this hypothesis must be carefully considered because of our limited knowledge of pyrolysis mechanisms. The possibility of phenol formation during the thermal fragmentation process from elimination reactions followed by cycliza-tion of poly conjugated chains has been suggested by Bracewell (22) and should be investigated. 

Silylene SiH2 was directly detected during the pyrolytic decomposition of silane (and ethylsilane EtSiII3) for the first time by intracavity laser spectroscopy in 198865. The high detection sensitivity of this experiment allowed the pyrolysis mechanism to be examined under conditions that optimized film growth rather than spectroscopic conditions. 

Even though free-radical reaction schemes best represent the pyrolysis mechanisms, simple nth-order models for the pyrolysis of the lighter feedstocks have been proposed. The range of orders is from 0.5 to 2.0, and significant differences are often reported in the literature. The overall activation energies vary from about 50 to 95 kcal/g mol. 

Scheme 3.1 Pyrolysis mechanism of ionic liquids containinig halide ...

L.F. Albright, W. Corcoran and B.L. Crynes (Eds.), Hydrocarbon Pyrolysis. Mechanisms and Industrial Applications, Academic Press, New York in press. 

The present authors found another pyrolysis mechanism leading to optical anisotropy (10), in which no definite liquid phase was observable during the carbonization. In some carbonaceous substances, such as semi-anthracite coal, optical anisotropy over broad regions develops promptly at certain temperatures from the highly viscous stage. The component layers appear to be stacked, and are rearranged by heat-treatment to show optical anisotropy. Such a mechanism can be called the "preordered layer-transformation mechanism". 

Species, 98 Reaction Pyrolysis Mechanism SCHEME II, CRAY-1, Optimized... 

D.K. Russel, Gas-Phase Pyrolysis Mechanisms in Organometallic CVD, Chemical Vapor Deposition, Vol.2,1996, pp.223-233. 

Mechanistic modeling has been useful in studying pyrolysis kinetics at low conversion (4,5,6). Few attempts have been reported at the high conversion levels of commercial cracking (7). This stems from the large number of species and free radicals and of their associated reactions, which increases substantially with conversion and leads to excessive computation time. In addition, when one considers that precise pyrolysis mechanisms, for even a simple feedstock such as propane (8), are still subject to dispute, it is clear that more empirical models will continue to dominate commercial applications. 

Thanks to this work, and subsequent work on pyrolysis mechanisms, it has become clear that three different types of elementary reactions are involved (see Chapter 2, Volume 2 for a discussion of chain reactions). 

Space does not permit a detailed consideration of the other paraffin hydrocarbons whose pyrolysis mechanisms have been elucidated the following are some references to recent publications ... 

Cellulose pyrolysis has been studied in detail from a variety of points of view mainly related to chemical utilization of wood pyrolysis products or to fire related problems. Analytical pyrolysis of cellulose is not often used as a tool for cellulose detection, but it is a common procedure for studying the pyrolysis products. A variety of analytical procedures have been applied for this study, pyrolysis/gas chromatography/mass spectrometry (Py-GC/MS) being the most common [11-16]. Besides Py-GC/MS, other analytical procedures also have been utilized, such as Py-MS [17,18], Py-IR [19], and off-line Py followed by HPLC [20]. The Py-MS spectrum of cellulose was shown in Figure 5.4.1 (B). Some procedures applied GC/MS on derivatized pyrolysis products (off-line), the derivatization being done by silylation [21], permethylation, perbenzoylation [22], etc. Information about cellulose also has been obtained from the analysis of pyrolysis products of several cellulose derivatives, such as O-substituted cellulose [23]. Also the study of cellulose crystalline structure with X-ray during pyrolysis has been used [23a] to generate information about the pyrolysis mechanism. 

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