Anna Marzec

The concept of a mobile or molecular phase within the macromolecular network of coals: A debate

Abstract The question addressed here is whether coals contain a substantial fraction of relatively small molecules clathrated within the macromolecular network and requiring long time or extra thermal energy for release. The debate primarily hinges on the interpretation of 1 H n.m.r. data, but some other relevant findings reported in the literature are recapitulated. Cases are made out by the original authors for two different interpretations of the n.m.r. data (it should be emphasized that there is no dispute about the actual data, only about its interpretation). It appears that there are two (or possibly three) populations of protons in coals, which exhibit appreciably different free induction decay times and hence have quite different levels of rotational mobility. This is a fact of considerable interest in its own right, but the possession of high mobility does not necessarily mean that the mobile protons must be in relatively small molecules which are free to tumble in cages within the macromolecular network. Taking into account evidence from extractability and other experiments, it is possible to draw the conclusion that the content of relatively small molecules held within coal is larger than has been supposed and is essentially equal to the content of substances containing highly mobile protons, though a substantial proportion of these substances can probably be extracted only under various special conditions. However, it should be stated that there is disagreement between the authors on the admissibility of evidence from solvent extraction experiments. There is certainly evidence that the very high extractability of lignites with ethylenediamine is due to chemical reaction, and it is argued that this may be true also of higher-rank coals.

Molecular structure of coals: A debate

Abstract Recent evidence concerning the molecular phase present in coals is discussed. It is proposed that pyrolysis-field ionization mass spectrometry (py-f.i.m.s.) provides information on the chemical nature of such material, but caution is also suggested in these interpretations. N.m.r. experiments can give information concerning the spatial location of molecular species, and domains with different mobilities can be recognised. The importance of considering coal origins and petrology in discussions of coal structure is emphasized. An empirical perspective of the molecular phase is afforded by solvent-free catalytic hydrogenation. Coal depolymerization by solvated electrons yields products consistent with polymethylene as a major part of the macromolecular network. The relation of these observations to the molecular mass of extracted material is suggested as crucial.

Bituminous coal extraction in terms of electron-donor and -acceptor interactions in the solvent/coal system

Experiments on high volatile bituminous coal extraction at ambient temperature have been carried out by means of 18 solvents having their electron-donor and -acceptor properties quantitatively determined (DN and AN numbers) by Gutmanns method. A model for coal extraction, based on the assumption that donor-acceptor bonds occur in coal and are responsible for binding together macromolecular network and extractable substances filling the pores of a network, has been worked out and verified on the basis of experimental data. The results lead to the conclusion that extraction is in principle, a substitution reaction: pore substances are replaced by a solvent molecule in their Donornetwork → Acceptorpore substance or Donorpore substance → Acceptornetwork bonds. Solvents capable of substitution are characterized by specific DN and AN values.

Towards an understanding of the coal structure: a review

Abstract This review is limited to studies that directly refer to the following concepts: (i) macromolecular structure of coal; (ii) the contribution of relatively low molecular substances in the structure; (iii) factors effecting capabilities of solvents in coal extraction and the extraction mechanism; as well as (iv) intermolecular forces occurring in coal organic matter. Although concept (i) was introduced in the 1970s, no great progress has been achieved. Available data for a few coals indicate that mass distribution of the macromolecular part are in the 1300–3500 amu range. There are no experimental data available that could describe the structure of the macromolecular part of coal. Referring to item (ii), the presence of significant amount of various substances in the ∼100–500 amu range in coals was evidenced by mass spectrometry methods (field ionization mass spectrometry (FIMS) and matrix-assisted laser desorption/ionization–time-of-flight (MALDI–TOF)). As for item (iii), major contributors that influence solvent extraction capability is the electron-donicity of the solvent as well as the shape and size of its molecules. One major step of the mechanism of coal extraction is the substitution phenomenon. Presumably (in reference to item (iv)), all known intermolecular forces hold together coal organic matter. However, only hydrogen bonds can be directly detected, as yet.

Molecular structure of bituminous coal studied with pulse nuclear magnetic resonance

Abstract High-volatile bituminous coal and coal saturated with fully deuterated solvents, were studied using the 1 H n.m.r. pulse technique. Free-induction decay (FID) signals were separated into Gaussian and Lorentzian components and contents of immobile and mobile phases of coal matter were calculated. Spin-spin relaxation times, T 2 , for both phases were determined. Some solvents caused a significant decrease in the content of the coal immobile phase. It is concluded that the immobile phase of coal consists of macromolecules and molecules. The latter are attached to macromolecules by electrondonor-acceptor interactions which can be destroyed by some solvents. The contents of macromolecules (60 wt%) and molecules (40 wt%) in the coal were determined.

Molecular components of coal and coal structure

Abstract A high-volatile bituminous coal was extracted at room temperature by various organic solvents. The yields of the extracts ranged from 4.5 wt% daf (ethanol/benzene extract) to 38 wt% daf (pyridine/ethylenediamine extract). The extracts were analysed by Fl mass spectrometry; the volatile part (75–80 wt%) was composed of substances of molecular weight in the range 70–800 amu, but the compounds in the 200–600 amu range predominated. Over 300 compounds were identified by high-resolution mass spectrometry. The results indicate that compounds

Application of 1H pulse n.m.r. to the determination of molecular and macromolecular phases in coals

Abstract 1 H n.m.r. free-induction decay (FID), after single 90 ° pulse excitation has been studied for bituminous and subbituminous coals and after saturation with deuterated pyridine and sulphuric acid. FID signals were broken down into Gaussian and Lorentzian components and their initial amplitudes determined. The ratio of hydrogen atoms occuring in the mobile phase (HMPh) and in the immobile phase (HIPh) were calculated. The ratios, HMPh:HIPh, measured in the presence of pyridine-d 5 and D 2 SO 4 delimit the range where the ratio of molecules:macromolecules occuring in coals can be found. The procedure described in this Paper can be used to estimate the total content of molecules (extractable and non-extractable) occuring in coals.

Macromolecular and molecular model of coal structure

This paper presents a review of experimental data which point out that coal organic matter consists of a cross-linked macromolecular network and a complex mixture of relatively small molecules. The components of molecular phase are assumed to be clathrated in the macromolecular phase by means of electron-donor—acceptor interactions. Some parts of the molecular phase may be not extractable due to restricted orifice sizes of pores created by a macromolecular network. Determination of the content of a molecular phase in coals seems to be solved by the application of 1H-NMR free-induction decay measurements. A comparison of calculated NMR line for Wisers representative structure with Gaussian component of the experimental NMR line for some low rank coals indicates that the above structure is a good representation for building blocks of the macromolecular phase of some coals.

Swelling of coal in relation to solvent electron-donor numbers

Abstract Twenty solvents characterized by electron-donor and electron-acceptor number and solubility parameter were used to study the swelling of a hvB coal. It was found that extent of swelling could be correlated with solvent electron-donor number. Mechanisms of swelling include solvent-induced breakage of electron-donor—acceptor interactions between coal macromolecules.

Mechanism of swelling and extraction and coal structure

Abstract An attempt is made to develop a comprehensive model of coal structure based on results of various experiments involving extraction and swelling of a coal by solvents. These are described in terms of electron-donor and electron-acceptor characteristics, of mass spectrometric analysis of coal extracts, of pulse 1 H n.m.r. measurements of mobilities of coal components, porosity measurements. The theory of coal swelling cannot be adapted from the theory of polymer swelling.