Other Pyrolyser Types

Besides the previously described pyrolyser types, some other pyrolysers have been constructed and reported in literature [16,30,32]. Some are based on variations of typical pyrolyser systems. One such system uses a microfumace pyrolyser with the capability to hydrogenate the pyrolysis products. For this purpose, the system uses hydrogen carrier, and, in line with the microfurnace, it has a catalyst column containing a solid support with Pt-catalyst (and a precolumn portion to trap non-volatile pyrolysis products) [31]. 

A different system utilizes as a source of heat an electric arc [30], but limited applications were reported for it, and also an infrared pyrolyser is manufactured [32]. Other techniques such as photolysis [33] were utilized for breaking down polymers for further analysis. However, these cannot be considered pyrolytic procedures. A theoretical approach has been developed [33] to compare mass spectrometric, thermolytic and photolytic fragmentation reactions. 

7 Comparison of Analytical Performances of Different Pvroivser Types. 

Comparisons between the results obtained using different pyrolysers are not uncommon in literature [16,34-36]. These comparisons have two objectives to assess the quality of the analytical results (reproducibility, sensitivity, etc.) of a certain type of pyrolyser and to indicate how the results of one pyrolyser can be compared to those of another type. 

The comparison is not always straightfon/vard because the analytical instrument at the end of the pyrolyser may play an important role regarding the quality of the data. A global view of different characteristics of the main pyrolyser types is given in Table 4.7.1. 

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Eliminations belong to one of the most diverse reaction types [76] and numerous solvent-free pyrolyses (sometimes quantitative melt reactions) provided useful syntheses [58,81-87]. However, quantitative solid-state eliminations are rare (examples are found in the halogenations of 110,112, and 114 (Scheme 12)). If an elimination reaction cannot be performed purely thermally or photo-chemically, usually a catalyst or other auxiliary has to be added and it is then no longer waste-free. 

The pyrolysis of tetrasulfones in general leads to lower yields than in the case of disulfones because there are more opportunities for recombination reactions and rearrangements. Nevertheless, there are a few examples for successful pyrolyses of this type that give products which are not or hardly accessible by other methods. Low yields therefore are acceptable in these syntheses. 

The results for Run 19 (Vycor glass reactor), Run 21 (alonized Incoloy 800 reactor), and Run 14 (coke-covered Incoloy 800 reactor) were similar to both the kinetics and type of products obtained. Although neither oxygen or hydrogen pretreatments were tried in Vycor glass or alonized Incoloy 800 reactors prior to acetylene pyrolyses, it is thought that such pretreatments would have little or no effect on acetylene reactions. This conclusion is based on such pretreatments prior to pyrolysis with other hydrocarbons in these two reactors. It has been concluded that all increases in acetylene conversions above those of Runs 14, 19, and 21 were in some way caused by surface reactions. Based on this assumption, surface reactions were of major importance in Runs 15, 18, and 23. 

There are several advantages of the resistively heated filament pyrolysers compared to other types. They can achieve very short TRT values, the temperature range is large, and Teq can be set at any desired value in this range. Several commercially available instruments are capable of performing programmed pyrolysis, and autosampling capability is also available (such as the CDS AS-2500). 

A factor that must be considered with furnace pyrolysers as well as with the other types of pyrolysers is the achieving of short TRT values. A slow sample introduction in the hot zone of the furnace will end in a long TRT. A poor contact between the sample and the hot source may also lead to long TRT, most of the heat being transferred by radiation and convection and not by conduction. However, fairly short TRTs in furnace pyrolysers were reported in literature [16,17]. 

The walls of the expansion chamber as well as those of the RF region must be inert (glass or gold-coated), and the expansion chamber should be heated (at moderate temperatures 150-200° C) to reduce condensation. Common problems for this type of pyrolyser are condensations on the cool portion of the system. On the other hand, heating the walls of the sample region (with resistors) may generate decomposition of the sample before pyrolysis. Also, the expansion chamber extends the time for the sample to be introduced in the mass spectrometer ion source and therefore the time... 

Similarly to Curie point Py-MS. the expansion chamber may be needed for buffering the emission of the pyrolysate and for allowing a longer time for the MS to acquire scans (spectra). Different types of lasers, combinations of lasers, or other experimental setups were reported [52. 53] as utilized in Py-MS. 

Another MS technique used in connection to pyrolysis is MIMS (membrane introduction mass spectrometry). MIMS is in fact a special inlet for the mass spectrometer, where a membrane (usually silicone, non-polar) lets only certain molecule types enter the Ionization chamber of the MS. This allows, for example, direct analysis of certain volatile organic compounds from air. The system makes possible the coupling of atmospheric pyrolysis to a mass spectrometer [61a] allowing direct sampling of the pyrolysate. Other parts of the mass spectrometer do not need to be changed when using MIMS. 

Direct pyrolysis of the DKPs corresponding to Pro-Val or Val-Pro was proven to generate the same types of compounds shown in Table 12.2.2. This is a proof that DKPs are the main primary pyrolysis products of these dipeptides. However, the pyrolysis of the dipeptides and their corresponding DKP does not generate identical pyrograms (chromatographic profiles of the pyrolysates). Therefore, some other processes may take place during the pyrolysis of each compound. 

A successful technique applied for the analysis of humin and humic acids was the pyrolysis with on line methylation followed by GC/MS analysis [6,9]. In one such study [9] humin deashed by treatment with HCI and FIF was pyrolysed and compared to humic acid obtained from the same soil, showing that humin contains larger amounts of carbohydrates and aliphatic compounds. This type of study also revealed the presence in the humin and humic acid pyrolysates of monocarboxylic acids with up to 32 carbon atoms, dicarboxylic acids, methoxymonocarboxylic acids with up to 26 carbon atoms, triterpenoid acids, etc. These compounds were not reported in other studies (e g. [2]) where the chromatographic separation did not allow the detection of compounds difficult to elute due to their high boiling point and polarity. 

Other studies on coal were performed using pyrolysis, such as the measurement of the level of sulfur containing compounds in coal [27,28], or evaluation of polynuclear aromatic hydrocarbons (PAH) in coal [29]. The generation of PAH in coal pyrolysis is an important issue, as some of these compounds are known to have carcinogenic properties. A list of PAHs identified in coal pyrolysates is given in Table 14.2.2. The yield of PAH in coal pyrolysate depends to some extent on the coal type but mainly on the pyrolysis temperature. The variation of PAH levels as a function of temperature for several bituminous coals is shown in Figure 14.2.3. The yields of other pyrolysis products of coal were also shown to be temperature dependent [30]. 

As in the case of wood, besides pyrolysate composition, smoke composition of other plant parts such as dry leaves has been the subject of different studies [34]. As plants may contain a variety of biopolymers and small molecules, some of them specific for a certain plant, smoke composition can be very diverse. This explains why certain particular types of smoke are related to specific plants and specific plant parts. As an example, the smoke associated with roasting coffee contains phenols and pyrazines generated from both biopolymers and small molecules. One such small molecule from coffee that generates by pyrolysis a variety of phenols is chlorogenic acid. 

Thermal decomposition of polyethylene in an inert atmosphere starts at about 280° C and occurs mainly following fragmentation and dehydrogenation reactions, the fragmentation being predominant at temperatures below 600° C (see Section 2.2). Hydrocarbons, from 2 carbon atoms up to 90 carbons, were identified in pyrolysates. Three types of fragment molecules are the most common, namely alkenes, alkanes, and a,(B-dienes. Traces of other hydrocarbons also are formed during pyrolysis. Some reactions typical for polyethylene pyrolysis are shown below ... 

The pyrolysate of polyacrylic-/nfer-nef-polysiloxane copolymer contains as main fragment molecules pyrolysis products similar to those of poly(butyl acrylate) and of poly(dimethylsiloxane (see Figure 6.7.8. and Section 16.1). The identification of fragments that would indicate sequences of other comonomers or any molecular connections between the two types of comonomer units was not possible. Other copolymers with acrylic acid as comonomer were studied using analytical pyrolysis. Among these are copolymers with special properties such as the copolymer with the formula shown below ... 

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