Pyrolysis products volatile molecules

One of the main problems of the pyrolysis technique is related to the low volatility of pyrolysis products arising from natural and some synthetic macromolecules. In fact, the polar acidic, alcoholic and aminic moieties are not really suitable for gas chromatographic analysis. Their poor volatility and their polarity cause a rather low reproducibility of the pyrograms, low sensitivity for specific compounds, and strong memory effects. Memory effects need to be borne in mind when the pyrolysis of polar molecules is performed. Polar pyolysis products may not be completely eluted by the gas chromatographic column, and... 

The composition of the pyrolysis products is primarily determined by the means of disintegration of the macromolecule to the molecules of gas, oil, and solid residue. Thus to anticipate the pyrolysis oil composition of a plastic material, the chemical composition and structure of the polymer and its thermal decomposition reactions should be consistently considered. Typical thermal decomposition pathways of the various polymer kinds are abundantly treated in the relevant scientific literature [18-20]. Thermal decomposition of the polymer component of a plastic material is expected to begin at the weakest chemical bonds of the macromolecule. However, there are decomposition pathways which require lower energy than the direct breakage of the bonds, when rearrangement over four or six neighbouring atoms leads to the elimination of a volatile compound or to the scission of the macromolecular chain. 

The DTA spectra show an exothermic peak at about 200 C and an endothermic one which starts at the same temperature of the weight lost. The exothermic peak in the first step is typical of crosslinking reactions while the endothermic one is due to the chain breaking with the probable formation of monomer or volatile molecules. Some typical pyrolysis products from two different polyamides are listed in Table 5.2. 

Although the applications described here have concerned diverse macromolecular samples, pyrolysis is also of utility in the analysis of smaller molecules, particularly quaternary ammonium compounds, which undergo quantitative thermal fragmentation to volatile products. For example, the specificity and sensitivity afforded by Py-GC/MS with mass fragmentography renders the technique suitable for the analysis of endogenous neurotransmitters such as acetylcholine at the l-2pmol level. However, in all cases, analytical throughput is limited by the retention times of the pyrolysis products, a condition that is particularly severe in taxonomic applications. Automation allows extended use, but direct Py-MS... 

Many of the reactions listed at the beginning of this section are acid catalyzed, although a number of basic catalysts are also employed. Esterifications are equilibrium reactions, and the reactions are often carried out at elevated temperatures for favorable rate and equilibrium constants and to shift the equilibrium in favor of the polymer by volatilization of the by-product molecules. An undesired feature of higher polymerization temperatures is the increased probability of side reactions such as the dehydration of the diol or the pyrolysis of the ester. Basic catalysts produce less of the undesirable side reactions. 

The volatile components from coal pyrolysis are primarily small hydrocarbons and oxygen-containing molecules. By adding H2O and limited O2 and while heating coal, these products incorporate considerable 0 atoms into the volatile products to form alcohols, aldehydes, ketones, and acids. However, these products consist of many molecules that... 

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