Mechanism of pyrolysis

G.R. Ponder and G.N. Richards, A review of some recent studies on mechanisms of pyrolysis of polysaccharides Biomass Bioenerg., 7,1 24 (1994). 

The mechanism of pyrolysis reactions of biomass was extensively discussed in an earlier study (Demirbas, 2000). Water is formed by dehydration. In the pyrolysis reactions, methanol arises from the breakdown of methyl esters and/or ethers from decomposition of pectin-like plant materials. Methanol also arises from methoxyl groups of uronic acid (Demirbas and Giillii, 1998). Acetic acid is formed in the thermal decomposition of all three main components of wood. When the yield of acetic... 

Such correlations have important bearings on the current level of understanding regarding the mechanisms of pyrolysis devolatilisation and char formation. Furthermore, standard forms of analyses are subject to oxidation or weathering effects and this study demonstrates that correlations between levels of unbumt carbon and vitrinite reflectance (33) maybe potentially misleading unless the affects of oxidation or weathering are considered. 

Mos of the solid carbonaceous material available to industry is derived from the pyrolysis of petroleum residues, coal, and coal tar residues. Understanding the reactions occurring during pyrolysis would be beneficial in conducting materials research on the manufacture of carbonaceous products. The pyrolysis of aromatic hydrocarbons has been reported to involve condensation and polymerization reactions that produce complex carbonaceous materials (I). Interest in the mechanism of pyrolysis of aromatic compounds is evidenced in a recent study by Edstrom and Lewis (2) on the differential thermal analysis of 84 model aromatic hydrocarbons. The study demonstrated that carbon formation was related to the molecular size of the compound and to energetic factors that could be estimated from ionization potentials. 

D. Price, A.R. Horrocks, M. Akalin, and A.A. Faroq, Influence of flame retardants on the mechanism of pyrolysis of cotton (cellulose) fabrics in air. J. Anal. Appl. Pyrolysis, 40 -1, 511-524 (1997). 

The detailed mechanism of pyrolysis of Cp iR compounds has been studied (307—313). A useful titanocycle is formed from Cp2TiCl2 and trimethylaluminum triethylaluminum gives a different product (314). The titanocycle adds to terminal olefins in the presence of 4-dimethylaminopyridine the adduct expels olefin above 0°C to yield a bistitanocyclobutane (315). The titanocycle can also behave like a Wittig reagent, reacting with aldehydes and ketones to give olefins (314,316). 

The study of kinetics and mechanism of pyrolysis is of considerable scientific interest, with regard to the thermal behaviour of organic molecules, the precise constitution of molecules or copolymers, including irregularities, such as structural defects and incorporation of initiator molecnles and radical scavengers, used to control the MW. 

The kinetic parameters given are derived from the thermogravimetric experiments. The mechanisms of pyrolysis, as discussed by this team, are too complex and varied to be treated here in more detail. 

There is no general agreement on the kinetics or mechanism of pyrolysis of 1-butene. Activation energies are reported in the range between 59.1 kcal/mol (8) and 71.5 kcal/mol (9), and reaction order (with respect to 1-butene) is reported to be 0.7 (8), 1.0 (9-12), approaching 2.0 at low pressures (13), and different than 1.0 at temperatures below 540°C (4). 

S. W. Benson, Mechanisms of Pyrolysis, Oxidation and Burning of Organic Compounds, NBS Spec. Publ. 357, U.S. Department of Commerce, 1972, p. 121. 

The remainder of this section will be concerned with a consideration of the mechanisms of pyrolysis of some of the simpler hydrocarbons. For reasons of space it is not possible to go into the more quantitative aspects of the kinetics the reader is referred to the individual papers that are cited, especially the more recent ones. 

These results provide additional confirmation for the mechanism of pyrolysis of simple polyolefins. The absence of monomer in the volatile products, the maxima in the rate curves, and the sharp decrease in the intrinsic viscosity for linear polymethylene (29) and polypropylene (2, 6, 13, 30) all point to an essentially random scission, due to pronounced intermolecular chain transfer, Equation 2. However, deviations appear when a, the fraction of bonds broken, or, what amounts to the same, the number average DP is examined as a function of time. For small a, the former relation should be one of simple proportionality and hnearity in 1/P. Instead, for both polypropylene (6) and polymethylene [see Figure 5, in (29)] curvature appears, indicating a reduction of the scission rate after an initial period of rapid degradation. For polypropylene this has been interpreted as a breaking of weak and normal bonds. Between 250° and 280° C., one weak link per 2.4 X 10 is found (6). At 295° C., the existence of more than two types of bonds would have to be postulated. 

The mechanism of pyrolysis of polystyrene was discussed in Section 2.1 in relation with the description of general mechanisms encountered in pyrolysis. The formation of traces of benzene, toluene, etc in the pyrolysate is also discussed in literature [15, 40, 61, 62]. Although they account for only a very small proportion of the molecules formed during pyrolysis, some side chain scissions may occur as shown below ... 

The mechanism of pyrolysis of starch199 and cellulose16,23,44 is very complex whether it is of heterolytic or radical nature has not yet been clarified, although the former alternative appears to be the more plausible. It has mainly been studied with cellulose182,193,196,205,213,214,230,232, 235,24i,246.26i-270 an(j can be described by the reactions given in Scheme 2. 

Li Xiao-min Lin Qi-zhao (2012). Maximum probability mechanisms of pyrolysis of com stalk. CIESC Journal. 63(8) 2599 2605. (In Chinese). 

The comparison of reactions (52) and (92) allows the deep-seated origin of the difference between the mechanisms of pyrolysis of neopentane (eqns 48 to 54) and of ethane (eqns 90 to 94), at "low" temperatures (lower than 800 K) to be understood. 

Two a priori lumped kinetic models were discussed in Chapter VIII the pH, YH model for pyrolyses and the COX and COLE model for oxidations. These models are basic models as they have the morphology which must be obeyed by all detailed reaction mechanisms of pyrolysis and oxidation. 

The above researches show that the conformational tautomerism of 1-indene imine intermediate plays an important role in the kinetic mechanisms of pyrolysis of quinoUne and isoquionUne. 1-indene imine intermediate determines the composition of the pyrolysis products to be the same, and also determines the total disappearance rates of the reactants to be the same whether the original reactant is quinoline or isoquinoUne. The intramolecular hydrogen migration is an important reaction step, which appears widely in the paths of the pyrolysis mechanism. 

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