When small amounts of a good hydrogen-bond acceptor such as pyridine or
THF are added to
a nonpolar swelling solvent such as chlorobenzene or toluene, there is
a very rapid increase in
coal swelling with increasing acceptor concentration which presently
almost levels off. When
swelling is plotted against pyridine concentration, the result is what
appears to be a titration
curve. This curve permits an estimation of the number of
hydrogen-bond cross-links in the coal.
We believe that this behavior is due to the selective association
between the hydrogen-bond
acceptor (e.g., pyridine) and hydroxyl groups which are cross-links
between macromolecular chains
in the coal. The selectivity of the acceptor for cross-linking
hydroxyls over other hydroxyls (more
favorable free energy for association of pyridine with the
cross-linking hydroxyl) is due to the
much more favorable entropy change which occurs when one of these
cross-links is disrupted by
formation of a new hydrogen bond to pyridine. This disruption
frees a portion of the coal to
adopt many more possible conformations lending to the favorable entropy
change. This model
leads directly to the prediction that this titration curve will be
independent of the hydrogen-bond acceptor, a prediction which is verified by the identical curves
observed using pyridine or
THF in chlorobenzene with Illinois No. 6 coal. Changing the
nonpolar solvent does not result in
a change in the number of hydrogen bonds in agreement with this model.
About 1/3 of the hydroxyl
groups in Illinois No. 6 coal form network-active hydrogen bonds
(cross-links) and there are about
1.7 hydrogen bonds per 100 carbon atoms. In the higher rank
Pittsburth No. 8 coal there are
approximately 0.3 cross-linking hydrogen bonds per 100 carbon
atoms.
The isosteric adsorption enthalpies of polar molecules on Illinois No. 6 coal have been measured by inverse gas chromatography (IGC), and the measured values have been separated into specific and van der Waals contributions by using adsorption heats on graphitized carbon black as a model for the van der Waals contributions. The specific adsorption heats of seven organic bases on Illinois No. 6 coal are identical to heats of hydrogen-bond formation with p-fluorophenol, suggesting that the bases hydrogen-bond to pendant phenol groups on the coal surface. Alcohols, amines, and pyridine have more exothermic specific adsorption heats on original (17 % ash) compared to demineralized (2% ash) coal, while organic oxygen bases have similar adsorption heats on both coals. All of these adsorbates have similar adsorption heats on citric acid demineralized and HF-HC1 demineralized Illinois No. 6 coal, demonstrating that pyridine, alcohols, and amines interact more strongly with carbonate and/or ion-exchangeable mineral matter, while oxygen bases interact preferentially with the organic component of the coal surface. Carbon dioxide and carbon disulfide both interact specifically (~2 kcal/mol) with the Illinois No. 6 coal surface. IGC data for hydrocarbons have been used to calculate reasonable values for the dispersive solid surface tension of Illinois No. 6 coal. Extracting Illinois No. 6 coal or heating it to 250 °C causes a decrease in the dispersive surface tension.
Inverse gas chromatography (IGC) has been used to obtain adsorption heats and entropies for hydrocarbons and noble gases on Illinois No. 6 coal. Plots of the natural logarithm of retention volume over temperature vs inverse temperature (van't Hoff plots) are linear. The y-intercepts of these plots decrease with increasing coal particle diameter, demonstrating that the retention volume is sensitive to the external coal surface. Adsorption heats for hydrocarbons on Illinois No. 6 coal heated to 150 °C are more exothermic than heats on graphitized carbon. Adsorption heats for hydrocarbons on Illinois No. 6 coal heated to 250 °C or extracted with tetrahydrofuran are similar to those on graphitized carbon. The same linear plot of adsorption heat vs adsorption entropy (isokinetic relationship) is observed for hydrocarbons on original and extracted Illinois No. 6 coal. The data demonstrate the potential of the technique for studying the surfaces of coals and modified coals.
Isosteric heats of adsorption for seven organic bases on demineralized Illinois No. 6 coal have been measured by inverse gas chromatography (IGC). These heats can be separated into nonspecific and specific interactions by using a previously established correlation between molecular polarizability and the enthalpy of nonspecific interactions with the coal.1 For all of the bases, the specific interaction heats are equal to the heats of hydrogen-bond formation between that base and p-fluorophenol.1 2 It appears that these bases associate with the coal surface by hydrogen-bonding to pendant phenolic hydroxyl groups.IGC is a dynamic adsorption technique used routinely
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