Other hydrocarbon pyrolyses

The kinetics of a number of other hydrocarbon pyrolyses have been studied some recent references are acetylene, Kistiakowsky et al. benzene, Hou and Palmer toluene, Takeuchi et al. 1-butene, Trotman-Dickenson et al. 2-methyl-1-pentene, Taniewski 4-methyl-1-pentene, Taniewski . 

The occurrence of molecular eliminations in photolysis is something that does not occur to a significant extent in the pyrolysis of hydrocarbons. Most frequently the molecular decomposition is accompanied by free-radical decomposition, with the fraction of each often dependent on the wavelength of the light used to initiate reaction. Such a case is the photolysis of methane where the following reactions occur  

In order to understand the photolysis completely it is necessary to distinguish between these two reactions and to assess each of the reactions separately this is not usually an easy matter. However, various techniques, particularly the examination of the isotopic distribution of products from the photolysis of a known mixture of a hydrocarbon and its fully or partially deuterated counterpart, have led to the understanding of the roles of the molecular and free-radical processes. 

From a succession of investigations of the photolysis of methane has emerged a fairly complete understanding of its mechanism. The earliest studies were inconclusive owing to incomplete product analysis. More recent studies ° have indicated that the major products are hydrogen, ethane, propane and ethylene with smaller quantities of acetylene, -butane, isobutane, propene, isobutene and allene. 

The elucidation of the reaction mechanism has been aided by the following important findings. 

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Historically, of the various hydrocarbon pyrolyses, that of ethane has been the object of the greatest amount of investigation. As a consequence more is known about it than about the pyrolysis of any other hydrocarbon. Even more important, sufficient thermal and kinetic data are now available so that the rates (and activation energies) of various chain mechanisms can be calculated with about order of magnitude reliability. 

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. 

Although methane is the simplest hydrocarbon, the elucidation of the mechanism of its pyrolysis has proved a matter of considerable difficulty. The reaction proceeds in a very different way from the other paraffin pyrolyses a carbon-carbon single bond is much weaker than a carbon-hydrogen bond so that C-C bond ruptures are important in all of the other pyrolyses. In the methane pyrolysis C-C bond rupture becomes gradually more important as product e.g. ethane) accumulates, so that the character of the process changes as reaction proceeds. 

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 pyrolysis results obtained using the Vycor reactor were used to make various comparisons. Increased temperatures and decreased partial pressures of entering propane both result in increased ethylene yields. Such findings are consistent with the general trends reported by many previous investigators for pyrolyses of other hydrocarbons and specifically by those (3,4) who have investigated propane pyrolyses in metal reactors. The ore-treatments of metal surfaces also affected product composition. Less surface reactions occur on H S-treated metal surface as compared to untreated surfaces. Oxy n-treated surfaces that have metal oxides on the surface tend to promote undesired surface reactions and to produce considerably more carbon oxides. 

Hydrocarbons and hydrogen halides are omitted since they will be dealt with elsewhere.) The chemical properties of most of these hydrides are rather well known, but this cannot be asserted for their decomposition kinetics. Some of them are very stable (H20, HF, NH3) while others decompose easily at room temperature (TeH2, PbH4). A study of the homogeneous decomposition has only been undertaken for those elements inside the frame in the Table. The pyrolyses of the others have either been found to proceed heterogeneously or the kinetics is unknown. 

The distribution of n-alkanes, isoprenoids and other branched hydrocarbons in the saturated/unsaturated hydrocarbons fraction of the modem microbial mat ranges mostly between C14 and C21, in extracts and pyrolysates (Figure 5A and 6C). It is similar to the distribution of the hydrocarbons described in the "top mat" of the Gavish sabkha in Israel (17). The main differences are the presence, in the extract of... 

I) and of Brown and Albright (2), who earlier studied surface reactions that occur during the pyrolysis of hydrocarbons. Such pyrolyses are used for commercial production of ethylene, other olefins, diolefins, and, to some extent, aromatics. Several important reactions occur on the inner surfaces of the high-alloy steel tubes used for pyrolyses. These surface reactions occur simultaneously and, to some extent, consecutively along with the gas-phase reactions that produce the desired products of... 

The thermal decompositions (pyrolyses) of hydrocarbons other than the cyclic ones invariably occur by complex mechanisms involving the participation of free radicals the processes are usually chain reactions. In spite of this, many of the decompositions show simple kinetics with integral reaction orders, and this led to the conclusion by the earlier workers that the mechanisms are simple. Ethane, for example, under the usual conditions of a pyrolysis experiment, decomposes by a first-order reaction mainly into ethylene and hydrogen, and the mechanism was thought to involve the direct split of the ethane molecule. Rice et however, showed that free radicals are certainly involved in this and other reactions, and this conclusion has been supported by much later work. An important advance was made in 1934 when Rice and Herzfeld showed how complex mechanisms can lead to simple overall kinetics. They proposed specific mechanisms in a number of cases most of these have required modification on the basis of more recent work, but the principles suggested by Rice and Herzfeld are still very useful. 

Among the compounds identified besides isoprene and its oligomers are several aromatic hydrocarbons. Also, a few fatty acids were identified. Low levels of aldehydes were detected in the fresh rubber latex, and the presence of the acids in the pyrolysate is not unexpected [9]. However, these acids may also come from contaminants in the pyrolysis experiment. The peaks corresponding to pentamers and hexamers were not obvious in Figure 6.1.3, possibly due to the separation conditions or due to a higher pyrolysis temperature. Some compounds other than those indicated in Figure 6.1.3 were reported to be present in natural rubber pyrolysate [4,10], but their detection may depend on specific pyroiysis conditions and on the sensitivity of the analytical procedure. 

One important industrial application of vulcanized rubber pyrolysis is related to the processing of used tires (which are generated worldwide at a rate of over 5 million tons per year). The shredded scrap tires are commonly pyrolysed between 450 and 600" C, generating char (37-38 wt.%), oils (53-58 wt.%), and gases (4-9 wt.%) [14, 15]. The gases are composed mainly of H2, CH4, C4H6, CO, CO2. Other aliphatic hydrocarbons were also detected in gases. The oils contain a mixture of hydrocarbons, with DL-limonene as a main component (see Table 6.1.2). However, of special interest was the... 

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 seen in Table 14.4.2, pyrolysis of some kerogens generates mainly hydrocarbons. However, the pyrolysate of some other kerogens have a much more complex composition than the Wild Harbor shale sample from Table 14.4.1 and 14.4.2. As an example, a sample from Cariaco Trench (dark olive calcareous clay 1.3 10 years old) [41] generates by pyrolysis at 610° C a variety of compounds that are listed in Table 14.4.3. 

The study of the matrix on pyrolysis result has an additional use besides the understanding of the origin of pyrolysate components. This is related to the influence of the matrix on the generation of specific hydrocarbons from a certain starting organic substrate under the infiuence of heat and of catalysis [46,47]. However, most of these studies are not directly related to analytical pyrolysis. In these studies, furnace pyrolysers were commonly preferred to small sample and flash pyrolysis [46]. These and other pyrolysis appiications for the study of kerogens and also of oil related components such as asphaltenes [47] have been proven extremely useful in practice [19]. 

Tobacco leaf has a complicated chemical composition including a variety of polymers and small molecules. The small molecules from tobacco belong to numerous classes of compounds such as hydrocarbons, terpenes, alcohols, phenols, acids, aldehydes, ketones, quinones, esters, nitriles, sulfur compounds, carbohydrates, amino acids, alkaloids, sterols, isoprenoids [48], Amadori compounds, etc. Some of these compounds were studied by pyrolysis techniques. One example of pyrolytic study is that of cuticular wax of tobacco leaf (green and aged), which was studied by Py-GC/MS [49]. By pyrolysis, some portion of cuticular wax may remain undecomposed. The undecomposed waxes consist of eicosyl tetradecanoate, docosyl octadecanoate, etc. The molecules detected in the wax pyrolysates include hydrocarbons (Cz to C34 with a maximum of occurrence of iso-Czi, normal C31 and anti-iso-C32), alcohols (docosanol, eicosanol), acids (hexadecanoic, hexadecenoic, octadecanoic, etc ). The cuticular wax also contains terpenoids such as a- and p-8,13-duvatriene-1,3-diols. By pyrolysis, some of these compounds are not decomposed and others generate closely related products such as seco-cembranoids (5-isopropyl-8,12-dimethyl-3E,8E,12E,14-pentadecatrien-2-one, 3,7,13-trimethyl-10-isopropyl-2,6,11,13-tetradecatrien-1al) and manols. By pyrolysis, c/s-abienol, (12-Z)- -12,14-dien-8a-ol, generates mainly frans-neo-abienol. 

The results for other conditions for polystyrene pyrolysis were reported. For example, pyrolysis on different catalysts was shown to lead to modifications of the yield of specific components in the pyrolysate. During the pyrolysis of PS on solid acid catalysts, the increase of contact time and surface acidity enhanced the production of ethylbenzene. Pyrolysis in the presence of water increases the yield of volatile products and that of monomer [30]. Studies on the generation of polycyclic aromatic hydrocarbons (PAHs) in polystyrene pyrolysates also were reported [36]. It was demonstrated that the content in PAHs in polystyrene pyrolysates increases as the pyrolysis temperature increases. The analysis of the end groups in polystyrenes with polymerizable end groups (macromonomers) was reported using stepwise pyrolysis and on-line methylation [46]. 

Similar to the other two examples, the alcohol, the corresponding aldehyde, and the hydrocarbon are found in large proportions in the pyrolysate. Also, phenol and the corresponding acid of the side chain substituent are present, similar to the examples of methyl and ethyl ethers. 

Pyrolysis process for poly(2-hydroxyethyl methacrylate) occurs similarly to that for other methacrylic acid esters. The formation of 2-methyl-2-propenoic acid 2-hydroxyethyl ester, the monomer, shows that unzipping is a significant part of the process. Some other compounds in the pyrolysate also are generated from the polymer cleavage, such compounds including 2-methyl-2-propenoic acid ethenyl ester, propanoic acid, 2-methyl-2-propenoic acid, ethanol, etc. On the other hand, some compounds are not expected in the pyrolysate and they can be impurities or additives. Examples of such compounds are the hydrocarbons (undecene, dodecane, 1-dodecene, etc.), the esters of ethylene diol and the free 1,2-ethandiol, etc. The initiator AIBN and its decomposition products 2-methyl-2-propenenitrile and 2-methylpropanenitrile identified in the pyrolysate show that the polymer was obtained using AIBN as initiator. 

Besides carbon dioxide, 2-propenoic acid, and acetic acid, all the other compounds seen in the pyrogram are alkanes and alkenes resulting from the side chain of the hydrocarbon monomer unit. A detailed inspection of the traces in the pyrogram shows very low levels of some of the same compounds formed during pyrolysis of poly(ethylene-alt-maieic anhydride) or poly(butylene-a/f-maleic anhydride), although they are not listed in Table 6.9.5. For example, some alkyl benzenes are present in the pyrolysate of poly(maleic anhydride-a/M-octadecene), but at about five times lower levels than in poly(butylene-a/f-maleic anhydride) pyrolysate, where alkyl benzenes were listed as trace. The mass ratio of maleic anhydride/butene is 1.75, while that of maleic anhydride/1-octadecene is about 0.39, which explains the lower presence of the... 

The selective production of methanol and of ethanol by carbon monoxide hydrogenation involving pyrolysed rhodium carbonyl clusters supported on basic or amphoteric oxides, respectively, has been discussed. The nature of the support clearly plays the major role in influencing the ratio of oxygenated products to hydrocarbon products, whereas the nuclearity and charge of the starting rhodium cluster compound are of minor importance. Ichikawa has now extended this work to a study of (CO 4- Hj) reactions in the presence of alkenes and to reactions over catalysts derived from platinum and iridium clusters. Rhodium, bimetallic Rh-Co, and cobalt carbonyl clusters supported on zinc oxide and other basic oxides are active catalysts for the hydro-formylation of ethene and propene at one atm and 90-180°C. Various rhodium carbonyl cluster precursors have been used catalytic activities at about 160vary in the order Rh4(CO)i2 > Rh6(CO)ig > [Rh7(CO)i6] >... 

As shown in Fignre II.B-2, pyrolysis of cholesterol la yields chrysene 111, Diels hydrocarbon IV —a methyl-cyclopentaphenanthrene—and nnmerous other PAHs. Both PAHs noted have also been identihed in pyrolysates of the major tobacco phytosterols [ Wynder et al. (4356), Van Dnnren (4022)]. More recently in the early 1970s, Hecht et al. (1560) discnssed the generation of chrysene and alkylchrysenes by pyrolysis of phytosterols. 

In 1958, Wynder et al. (4355) described the effect of varying the pyrolysis temperature on the yield of pyrolysate from an n-hexane extract from tobacco. The extracted material constituted 5.4% of the original tobacco weight and consisted of long-chained saturated and unsaturated aliphatic hydrocarbons, glycerides and other esters, solanesol and phytosterols and their esters, long-chained aliphatic esters, and a-tocopherol. Major findings from their study included (see also Table XXV-2) ... 

Several PAHs other than chrysene and Diels hydrocarbon (Fignre XXV-1) were subsequently identihed in sterol pyrolysates. In 1959, Wynder et al. (4355,4356) reported that PAHs were generated at both temperatures when tobacco sterols were pyrolyzed in air at 720°C and 850°C. At these temperatures, the pyrolysates constituted 28% and 22%, respectively, of the phytosterols pyrolyzed B[a]P constitnted... 

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