Acetaldehyde pyrolysis activation energy

Decomposition. Acetaldehyde decomposes at temperatures above 400°C, forming principally methane and carbon monoxide [630-08-0]. The activation energy of the pyrolysis reaction is 97.7 kj/mol (408.8 kcal/mol) (27). There have been many investigations of the photolytic and radical-induced decomposition of acetaldehyde and deuterated acetaldehyde (28—30). 

All these reactions are endothermic and have high activation energies. The observation that acetaldehyde concentration rises during the pic darret while that of formaldehyde decreases suggests that only the last reaction is important and that formaldehyde is formed by oxidation (e.g., of CH3) rather than by pyrolysis. 

Amorphous and semi-crystalline polypropylene samples were pyrolyzed in He from 388°-438°C and in air from 240°-289°C. A novel interfaced pyrolysis gas chromatographic peak identification system was used to analyze the products on-the-fly the chemical structures of the products were determined also by mass spectrometry. Pyrolysis of polypropylene in He has activation energies of 5-1-56 kcal mol 1 and a first-order rate constant of JO 3 sec 1 at 414°C. The olefinic products observed can be rationalized by a mechanism involving intramolecular chain transfer processes of primary and secondary alkyl radicals, the latter being of greater importance. Oxidative pyrolysis of polypropylene has an activation energy of about 16 kcal mol 1 the first-order rate constant is about 5 X JO 3 sec 1 at 264°C. The main products aside from C02, H20, acetaldehyde, and hydrocarbons are ketones. A simple mechanistic scheme has been proposed involving C-C scissions of tertiary alkoxy radical accompanied by H transfer, which can account for most of the observed products. Similar processes for secondary alkoxy radicals seem to lead mainly to formaldehyde. Differences in pyrolysis product distributions reported here and by other workers may be attributed to the rapid removal of the products by the carrier gas in our experiments. 

A sequence was proposed in Problem 3.4.4 for the pyrolysis of acetaldehyde. Predict the apparent activation energy of the overall reaction. 

The series of elementary steps below has been proposed to describe acetaldehyde pyrolysis to methane and CO. Derive the steady-state rate expression making the usual long-chain approximation i.e., that the chain length is very high. What comprises the apparent activation energy ... 

Chien and Kiang [44] carried out oxidative pyrolysis of PP at temperatures between 240 °C and 289 °C. The products were separated by GC and identified online by an interface GC peak-identification system. The major products were CO, H O, acetaldehyde, acetone, butanal, formaldehyde, methanol and other ketones and aldehydes. These identifications were confirmed by MS. Most of the products can be accounted for by well-known reactions of alkoxyl and peroxyl radicals the major products are derived from the secondary alkoxy and peroxy species. Oxygen starvation is manifested in diffusion-limited products of olefins and dienes, and the increase in the formation of CO and H O in an atmosphere of pure oxygen. The first-order rate constant at 240 °C is 2.4 x 10 Vs, with an overall activation energy of approximately 16 kcal/mol (67 kj/mol). If one assumes that the oxidative pyrolysis shares the same reaction pathways as autoxidation at lower temperatures, then the observed rate constants and activation energy may be calculated from kinetic... 

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