Alkanes distribution with carbon number

Briefly, n-alkane distributions with carbon number maxima in the 17-21 range (Cj y-C j ) originate largely from aquatic algal sources, while maxima characterized by higher carbon numbers are... 

For this purpose selectivities of hydrocracking of the n-alkanes are expressed in terms of probabilities of overall cracking reactions. The latter are calculated from the carbon number distributions of the cracked products (n. = number of moles of cracked products with carbon number i, from Figures 7, 8 or 9). Examples for the mode of calculation are represented in the following scheme which is valid for pure primary cracking. 

Vacuum Pistillate Fractions. Pyrolysis of the saturate fraction for 30 minutes at 450°C gave the n-alkane and 1-alkene product distribution shown in Figure 3. This same data in tabulated form is compared in Table I with the starting material. The comparison indicates a net loss in concentration for the n-alkanes C16 and larger and a net gain for those with 14 or less carbons. The 1-alkenes exhibit a similar relationship with carbon number. For this pyrolysis time, the yield of small... 

Another factor commonly used as a source indicator with respect to alkanes is the carbon number distribution. 

Grimalt J., Albaiges J. (1987) Sources and occurrence of C12-C22 re-alkane distributions with even carbon-number preference in sedimentary environments. Geochim. Cosmochim.Acta 51, 1379—84. 

Fig. 6-3. Alkanes in marine air near the ocean surface. Shown is the distribution of concentration with carbon number according to data of Rudolph and Ehhalt (1981) for C2-C5 alkanes over the Atlantic Ocean and of Eichmann el al. (1979, 1980) for C,-C28 alkanes in the air of the North Atlantic and South Indian Oceans.

Hydrocarbons found in the environment are of diverse structure and are widely distributed in the biosphere, predominantly as surface waxes of leaves, plant oils, cuticles of insects, and the lipids of microorganisms. Straight-chain HC, or alkanes, with carbon number maxima in the range of C17 to C21 are typically produced by aquatic algae. Conversely terrestrial plants typically produce alkanes with C25 to C33 maxima. Plants also synthesize aromatic HC such as carotenoids, lignin, aUcenoids, terpenes, and flavenoids. Polycyclic... 

The chromatograms of the liquid phase show the presence of smaller and larger hydrocarbons than the parent one. Nevertheless, the main products are n-alkanes and 1-alkenes with a carbon number between 3 to 9 and an equimolar distribution is obtained. The product distribution can be explained by the F-S-S mechanism. Between the peaks of these hydrocarbons, it is possible to observe numerous smaller peaks. They have been identified by mass spectrometry as X-alkenes, dienes and also cyclic compounds (saturated, partially saturated and aromatic). These secondary products start to appear at 400 °C. Of course, their quantities increase at 425 °C. As these hydrocarbons are not seen for the lower temperature, it is possible to imagine that they are secondary reaction products. The analysis of the gaseous phase shows the presence of hydrogen, light alkanes and 1-alkenes. 

At low temperature (375 and 400 °C), the product distribution obtained with the catalysts is very different from the one obtained under thermal cracking. With the catalytic cracking (ZSM-5), the obtained products are mainly n-alkanes, isomerised alkanes and alkenes with a carbon number between 1 to 6 whereas with the thermal cracking the whole range of n-alkanes with 1 to 9 carbon atoms and the 1 -alkenes with 2 to 10 carbon atoms are observed. This difference of product distribution can easily be explained by the cracking mechanisms. In one hand, the active intermediate is a carbocation and in the other hand it is a radical. 

Figure 7. Hydrocracking of n-alkanes with an even carbon number. Distribution of the cracked products.

This picture explains the great differences in product distributions of ideal hydrocracking and catalytic cracking. Furthermore, it is in agreement with the observation that carbon number distributions are similar in ideal hydrocracking of n-alkanes and in catalytic cracking of n-alkenes with the same carbon number (19). 

The saturated hydrocarbon distributions of the marl samples are dominated by long-chain n-alkanes of higher land plant origin (21) with a strong odd-over-even carbon number predominance. Hopanoid hydrocarbons are the next most abundant constituents, but other hydrocarbons particularly abundant in the laminite samples described hereafter are also clearly recognizable. 

A more critical evaluation of the above mentioned ratios and phenomena reveals the usefulness of the various palaeosalinity indicators. Distribution patterns of methylated chromans and the relative abundance of gammacerane are not influenced by sulfur incorporation reactions and may directly reflect species distributions in the palaeoenvironment. To some extent this holds for 14a(H),17a(H)/140(H),170(H)-steraneratios as well, although incorporation of sulfur may influence this ratio and original A7/A5-sterol ratios do not always correlate with hypersaline environments. The isoprenoid thiophene ratio is highly useful as a palaeosalinity indicator since the distribution of the C20 isoprenoid thiophenes directly reflects the distribution of their substrates. The other parameters (pristane/phytane ratio, odd-over-even carbon number predominance of n-alkanes, relative abundance of C35 hopanes and/or hopenes) should be used with caution because they obviously depend on the quenching by sulfur of specific lipids, a process which is not restricted to hypersaline environments. 

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