Aliphatic hydrogen content

Working co-operatively with others, we have found some indication that certain alilphatic linkages between aromatic nucleii are involved in the rapid dissolution of coal. The absolute aliphatic hydrogen content as determined by P. Solomon using FTIR (22) shows a very good linear relationship with conversion of coal in 3 minutes to pyridine soluble materials (Figure 14a). 

Figure 14a. Response of coal conversion to aliphatic hydrogen content...

For each coal, at the maximum in hydrogen content, or H/C atomic ratio, the aliphatic hydrogen content (determined by H NMR analysis) accounted for over 90% of the total hydrogen. The aliphatic hydrogen contents were 10.5% for the subbituminous coal,PSOC-1403, and 6.9% for the bituminous coal, PSOC-1266. The high aliphatic hydrogen content was associated with the presence of polymethylene chains. The early release of paraffinic material, as n-alkanes and as long chain substituents to aromatic structures, under conditions of mild pyrolysis has been observed in other research (13-15. ... 

An interesting comparison can be made on the basis of the hydrogen distribution of the three macerals. The aromatic hydrogen content (%H.r) has been calculated from the total area of the infrared absorption between 11 and 14 n (aromatic HCC rocking vibrations) (6). The aliphatic hydrogen content (% H.i) has been derived from the absorption of the aliphatic H-C stretching vibration between 3.3 and 3.6 m (7). The values found for the three macerals under investigation are reported in Table II, columns 3 and 4. 

In addition to qualitative analysis, a semiquantitative analysis of selected spectral regions can be carried out from the IR absorption bands [234,246]. In this approach the ratio of the aliphatic hydrogen content to aromatic carbon (Hai/Car) providcs a measure of the evolution of aliphatic structures, while the methyl-to-methylene ratio (CH3/CH2) is considered as an estimation of the length of the aliphatic chains. The ratios are calculated by integration of the peak areas or measurements of the peak intensities, often referred to as an internal standard. 

Only a limited number of coal-denved pitches were examined by H NMR because of their low solubility in solvents commonly used m conventional proton magnetic resonance. Table 12 reports the distribution of hydrogen for three of the pitches. Unlike coal-tar pitches, which typically have over 85% of the hydrogen bonded to aromatic carbon, the matenals listed in Table 12 are characterized by a high content of aliphatic hydrogen. 

The process is conducted at 700 °C. It yields semicoke, which is popular as a smokeless domestic fuel. It can at times be used in boiler also to avoid smoke. Yield of coke oven gas is less, of tar high, and of ammonia less. Calorific value of coke oven gas generated is more. The process produces aliphatic natured tar. Following carbonization the coke discharging process is difficult as it swells extensively but does not shrink much upon carbonization. Free carbon in tar (produced from the cracking of hydrocarbons) is less Coke produced is weaker. Volatile matter in the coke produced is more. Hydrogen content in the coke oven gas is less. 

Alkylation of solvent represents still another pathway for changing the properties of a recycle solvent. If we consider alkylation in terms of the transfer to methyl groups from coal to solvent components, then there are several structural and physical changes that occur to the solvent. Alkylation will increase the hydrogen content of the solvent at the expense of coal since the solvent molecule will have a C-H replaced by C-CH. This represents an increase in the aliphatic content and conversely a decrease in the aromatic content of the solvent. Kleinpeter (7) has indicated that alkylation of condensed aromatics is a problem. High aliphatic character will decrease the ability of the solvent to act as a physical solvent for coal liquefaction products. 

Solomon (16,has uset a different method to obtain extinction coefficients. Essentially, total hydrogen content from elemental analysis and hydroxyl content from measurements of the area of the 0-H stretching band near 3450 cm were used in conjunction with the peak areas of aliphatic and aromatic bands to obtain a plot from which extinction coefficients can be determined. In principle, this approach appears to be sound, but there are a number of problems. One difficulty, discussed above, is general to all infrared methods that have been employed so far what errors are introduced by summing peak areas over a number of bands, each of which has an individual extinction coefficient, and essentially averaging such coefficients for the total area Other problems involve the correct use of curve resolving techniques and the measurement of hydroxyl groups, which we will now consider in more detail. 

In general, the CRAMPS NMR spectra of fossil fuel materials do not exhibit a high degree of resolution this is because of the overlap of the multitude of resonances from the different H types in these complex materials. CRAMPS spectra of 10 coals ranging in rank from lignite to anthracite are shown in Fig. 19. " These spectra illustrate the resolution typically obtained for carbonaceous materials. Estimates of the aliphatic and aromatic hydrogen contents can be obtained, but generally require computer-aided spectral deconvolution. 

Nuclear magnetic resonance spectroscopy has also been used to derive structural parameters for the distribution of aliphatic hydrogen in coal and its extraction products. A ratio of four methylene groups per methyl group was noted also, the methyl group content was higher in the soluble material and varied with the solvent power of the solvent. 

Hydrocarbons, compounds of carbon and hydrogen, are stmcturally classified as aromatic and aliphatic the latter includes alkanes (paraffins), alkenes (olefins), alkynes (acetylenes), and cycloparaffins. An example of a low molecular weight paraffin is methane [74-82-8], of an olefin, ethylene [74-85-1], of a cycloparaffin, cyclopentane [287-92-3], and of an aromatic, benzene [71-43-2]. Cmde petroleum oils [8002-05-9], which span a range of molecular weights of these compounds, excluding the very reactive olefins, have been classified according to their content as paraffinic, cycloparaffinic (naphthenic), or aromatic. The hydrocarbon class of terpenes is not discussed here. Terpenes, such as turpentine [8006-64-2] are found widely distributed in plants, and consist of repeating isoprene [78-79-5] units (see Isoprene Terpenoids). 

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