Alkanes, pyrolysis

Rice—Herzfeld mechanism for alkane pyrolysis is an example which, in its... 

The reaction mechanism used for this study was the alkane pyrolysis scheme of Edelson and Allara. [10] This consists of 98 reactions involving 38 chemical species. It is not so large that it overburdens the computers used yet is of a size sufficient to yield meaningful timing of the program modules. 

For the alkane pyrolysis, two main reaction paths are possible (see also Section 2.2), one with dehydrogenation and the other with fragmentation. The free enthalpies for these reactions are as follows ... 

One of the early controversies in the field of pyrolysis revolved about the question of the importance of concerted, molecular elimination reactions of hydrocarbons to give olefins and alkanes as products. Numerous experiments (51) have given a decisive answer to this problem in favor of the completely radical mechanism for alkane pyrolysis. At the same time, ironically, very clear cut evidence has been elicited to show that both mono- and polyolefins can react quite readily in concerted, molecu-... 

Additional structural information can also be derived by Py-GC. Information on stereoisomerism, crystallinity, and sequence distribution data is sometimes able to be obtained. Isotactic and syndiotactic polypropylene may be identified by the ratio of oligomeric components. High- and low-density polyethylene can also be determined by the proportion of branch chain alkane pyrolysis products using a novel pyrolysis hydrogenation technique. 

Edblom, E. C. Orban, M. Epstein, I. R. 1986. A New lodate Oscillator The Landolt Reaction with Ferrocyanide in a CSTR, J. Am. Chem. Soc. 108, 2826-2830. Edelson, D. Allara, D. L. 1980. A Computational Analysis of the Alkane Pyrolysis Mechanism Sensitivity Analysis of Individual Reaction Steps, Int. J. Chem. Kinet. 12, 605-621. 

Other examples of such protocols exist in pyrolysis and combustion, where again a whole range of gas-phase organic mechanisms may be present depending on the starting fuel. An example of the likely reaction classes for alkane pyrolysis is given in Table 3.1 and for alkane oxidation in Fig. 3.2. A more detailed discussion of the... 

Edelson, D., Allara, D.L. A computational analysis of the alkane pyrolysis mechanism sensitivity analysis of individual reaction steps. Int. J. Chem. Kinet. 12,605-621 (1980)... 

In contrast, the ultrasonic irradiation of organic Hquids has been less studied. SusHck and co-workers estabHshed that virtually all organic Hquids wiU generate free radicals upon ultrasonic irradiation, as long as the total vapor pressure is low enough to allow effective bubble coUapse (49). The sonolysis of simple hydrocarbons (for example, alkanes) creates the same kinds of products associated with very high temperature pyrolysis (50). Most of these products (H2, CH4, and the smaller 1-alkenes) derive from a weU-understood radical chain mechanism. 

The alkanes have low reactivities as compared to other hydrocarbons. Much alkane chemistry involves free-radical chain reactions that occur under vigorous conditions, eg, combustion and pyrolysis. Isobutane exhibits a different chemical behavior than / -butane, owing in part to the presence of a tertiary carbon atom and to the stability of the associated free radical. 

Pyrolysis of alkanes is referred to as eraeking. Alkanes from the paraffins (kerosene) fraetion in the vapor state are passed through a metal ehamher heated to 400-700°C. Metallie oxides are used as a eatalyst. The starting alkanes are broken down into a mixture of smaller alkanes, alkenes, and some hydrogen. 

Homologous n-a-olefins by pyrolysis of high molecular weight C20-C30 n-alkanes (wax cracking)... 

Denig [118] has reported that the length and structure of the hydrocarbon chains of AOS may be determined by cleavage of the sulfo groups in presence of excess phosphorus pentoxide and hydrogenation of the hydrocarbons before determining the -alkanes and isoalkanes by pyrolysis gas chromatography. 

It is now clearly demonstrated through the use of free radical traps that all organic liquids will undergo cavitation and generate bond homolysis, if the ambient temperature is sufficiently low (i.e., in order to reduce the solvent system s vapor pressure) (89,90,161,162). The sonolysis of alkanes is quite similar to very high temperature pyrolysis, yielding the products expected (H2, CH4, 1-alkenes, and acetylene) from the well-understood Rice radical chain mechanism (89). Other recent reports compare the sonolysis and pyrolysis of biacetyl (which gives primarily acetone) (163) and the sonolysis and radiolysis of menthone (164). Nonaqueous chemistry can be complex, however, as in the tarry polymerization of several substituted benzenes (165). 

Recently, researchers have detected 2,5-dimethylfuran and 2-methylfuran and normal alkanes in kerogen of the 2.7 x 109 year old Belingwe, Rhodesia stromatolites, by the method of pyrolysis/ GC/MS [26]. They concluded that although furans could probably be derived from many compounds, their probable origin is in bacterial and algal sugars, and that the alkanes are either products of decarboxylation of fatty acids or unaltered constituents of ancient organisms. 

Based on the quantitative determination of pentadecane it was calculated that—with a sample load of about 200pg—alkanes are detected by this screening method if their concentration is 5ppm or more. It is obvious that highly volatile compounds when present as such (e.g. dioxane) cannot be measured quantitatively because considerable losses of such components occur during the evaporation of the suspension liquid from the pyrolysis wire when it is prepared. Quantities measured for such compounds must therefore be considered as minimum values. 

Mansuy et al. [97] investigated the use of GC-C-IRMS as a complimentary correlation technique to GC and GC-MS, particularly for spilled crude oils and hydrocarbon samples that have undergone extensive weathering. In their study, a variety of oils and refined hydrocarbon products, weathered both artificially and naturally, were analyzed by GC, GC-MS, and GC-C-IRMS. The authors reported that in case of samples which have lost their more volatile n-alkanes as a result of weathering, the isotopic compositions of the individual compounds were not found to be extensively affected. Hence, GC-C-IRMS was shown to be useful for correlation of refined products dominated by n-alkanes in the C10-C20 region and containing none of the biomarkers more commonly used for source correlation purposes. For extensively weathered crude oils which have lost all of their n-alkanes,it has been demonstrated that isolation and pyrolysis of the asphaltenes followed by GC-C-IRMS of the individual pyrolysis products can be used for correlation purposes with their unaltered counterparts [97]. 

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