Coal number average molecular

However, as pointed out above, the commonly proposed free radical mechanism is not entirely consistent with the observed behavior of H-donor solvents and coal. Further, a thermally promoted C-C or C-0 bond-scission is inconsistent with our observations in the -PrOH work at 335°C. As also mentioned, a major fraction of the coal was converted in this system to a product with a number-average molecular weight of less than 500. If we consider that the rate constant for the unimolecular scission of the central bond in bibenzyl is expressed (5) as... 

Table I summarizes comparisons between asphaltenes derived from bituminous coal liquefaction and those derived from petroleum crudes. The molecular size and atomic H/C ratios suggest a molecular profile quite different for the two asphaltenes. The ranges represent, as best as could be found, reasonable extremes for each of the properties. We are well aware that the number-average molecular weight of petroleum asphaltenes has been influenced by aggregate formation. To overcome this effect, molecular weight determinations should be made in dilute noninteracting solvents (e.g., methylene chloride), and solutions should be filtered or ultracentrifuged in helium-degassed solvents.

The reaction is second order, first order in phenol and first order in the methylene-bridged aromatic (27), Presumably the mechanism involves ipso protonation of the aromatic followed by attack of phenol on the cationic intermediate (27). When applied to coals, it renders them almost completely extractable into pyridine, and the number average molecular weights of the extractable material generally range between 300 and 750 (24). 

Bruceton coal was depolymerized in refluxing phenol using p-toluenesulfonic acid (HOTs) catalyst (29). The products were 91% extractable into pyridine and had a number average molecular weight of 400 in pyridine, measured by vapor pressure osmometry. Yet most of the materials in this solution have molecular weights above 2500. 

This technique has been used to estimate the molecular weight distribution in coal but the (number average) molecular weights (Mn) of the material soluble in pyridine after depolymerization can be affected by the presence of colloidal material. Whilst the reported value may be in the region of 400, removal of the colloidal material (by, say, centrifugation) may increase this value to ca. 1000. 

It was originally anticipated that the reaction would not cleave coal molecules but an examination of the molecular weight distribution of the products from the ethylation of coal (carbon content 90% w/w) produced data which indicated a number average molecular weight of the order of 3300 (Figure 10.21) (Sternberg et al., 1971). 

Coal tar pitch and the pyridine-insoluble fractions were examined by LD-MS [32,33] to define the upper mass limits of the mass spectra in terms of (1) signal noise ratio above a selected limit (5 1), (2) when the calculated weight and number average molecular masses no longer increased with increasing values of the upper... 

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