Cross-linked structure in coal

Figure L Proposed cross-linked structure in coals Aromatic ring cluster ( 3) connecting bond (—) tetra functional cross-link (A) multifunctional crosslink (B).

LUCHT AND peppas Cross-Linked Structures in Coals... 

George Kapo At the bottom of the bore holes there was a slight increase in the moisture content of the coal. In other words, with an increase in depth there is a minimum in moisture content. Is this minimum caused by a minimum in the cross-linked structure of the coal here ... 

As suggested by Honda the decrease in reactivity in his acidic oxidation with increase in rank below 78 and between 90 and 93% carbon for his coals is caused by increasing cross-link structures here. This can also explain the decrease in reactivity above 90% carbon for the acid permanganate results. The increase in reactivity noted by Honda going from 93% carbon to graphite is caused by ability of the concentrated phosphoric acid to disperse graphitelike structures. 

Generally, coal should be dried before extraction in order to determine extract yields and material balances however, this may collapse pore structure in low-rank and produce other changes in high-rank coals. Even drying at 100°C (212°F) can induce cross-linking reactions in low-rank coals, which reduce the tendency of the coal to swell in solvents such as pyridine. 

Authors concluded, that although the swelling degree is not directly connected with molecular characteristics of absorbed liquids, however determining factor is their parameter of solubility in spite of the fact that at detailed consideration of the dependencies Q =f(3) (or f(32)) there are a number of deviations (as same as in the work [5]) from the ideal curve for many solvents. It is necessary to notify that although it is hard to estimate the verisimilitude of determined in such a way molecular weights of structural links of a coal between the points of cross bonds, however, in a case of synthetic polymers in a same way determined masses of links visible don t agree with the values obtained in accordance with others methods. 

In many ways, the molecular models that we have used as the basis for our CAMD studies describe coal structure very well. However, none of the models investigated thus far contains explicit three-dimensional covalent cross-links. Actualfy, the models we have studied are primarily constructed of long chains of one-dimensionally-linked clusters with a number of short side-chains. However, it has been established on the basis of solvent swelling studies (11.15.16) that bituminous coal is primarily made up from a three-dimensional network of clusters held together by covalent bonds and by an even higher density of hydrogen bonds. These macromolecular models of coal, which are less concerned with the molecular structure than with the ways that clusters are bonded to one another, provide a complementary way of describing coal structure. 

More-complex models have been proposed to link the coal structure with volatile decomposition. The model, which considers the changes of functional groups and reactions of char-forming repolymerization (cross-linking), can predict gas species and tar in volatiles.34 36 Another model applies concepts of polymer decomposition to describe the release of tar fragments from the coal macromolecule.37 The models will not be summarized here since they are more complex than the single-step, two-step, and multiple-step models. Interested readers should consult the appropriate literature for more details. 

A majority of the proposed models suggest coals to consist of several ring aromatic and hydroaromatic structural units, cross-linked through aliphatic and ether bridges to form the three dimensional structure and in the pores and cavities of this structure reside weakly linked smaller molecules which are easily extracted by solvents. The model proposed by Solomon and Shinn are representative examples of this macromolecular model. 

However, these hypochlorite studies have been criticized on the basis of the claim that sodium hypochlorite only oxidize sjp carbon under the reaction conditions. For example, there is the claim that coal oxidation, by sodium hypochlorite under rigidly controlled experimental conditions, produces data which show that aromatic systems also undergo oxidation by hypochlorite. Thus, there are legitimate question about the occurrence of adamantane-type systans in coal. It is suggested that the main structural unit of bituminous coal is a three-dimensional cross-linked systan (Mayo, 1975 Lucht and Peppas, 1981) which contains condensed aromatic, as well as aUcycUc, systems that are connected by ether (or thioether) linkages as well as by methylene (or polymethylene) linkages. 

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