Dynamic nuclear polarization 13C n.m.r. of coal

A novel n.m.r. method for determining the aromaticity of coal has been evaluated. Polarization transfer from the paramagnetic electrons is used to enhance the intensity of the ¹³C n.m.r. spectrum. Analysis of five samples of Gondwana bituminous steam coals shows that the aromaticity of inertinite is far greater than that of vitrinite and that variation of maceral composition is a major factor determining coal aromaticity. This has been confirmed by the study of maceral concentrates.     

Quantitative aspects of solid state 13C n.m.r. of coals and related materials

The quantitative aspects of cross-polarization (CP), which is used in conjunction with dipolar decoupling and magic-angle rotation to obtain high resolution ¹³C n.m.r. spectra of coals, have been studied using a bituminous coal (82 wt% C, dmmf basis) and asphaltenes from an extract of the same coal. The condition for obtaining reliable quantitative data, that rotating frame ¹H relaxation times (T_1p′ these govern the extent of CP) are much longer than the time required to polarize the carbons present (≈1 ms), was met for the asphaltenes. In contrast, about half the protons in the coal have T_1p5 of ≈ ? 1 ms, these times being too short to allow CP of all the carbons. Although the aromaticities obtained for this coal were fairly constant (≈0.75) using (CP) contact times > 0.5 ms, the total peak intensity decreased markedly as the contact time was increased and was much less than that for the asphaltenes. These results indicate that not all the carbons in bituminous coals are observed by CP and, as a consequence, aromaticities reported in the literature for some bituminous coals appear to be low.     

Preliminary studies on the aromaticity of Australian coals: Solid state n.m.r. techniques

The aromaticities of samples from nine Australian coal seams, including pairs of hand-picked vitrains and durains, have been determined by ¹³C cross-polarization n.m.r. spectroscopy with magic angle sample spinning. The results clearly show that the aromaticity (f_a) of the coals increases with increase in vitrinite reflectance and carbon content and decrease in atomic H/C ratio. For a given coal seam, durain (inertinite-rich coal) has a higher f_a value than vitrain (vitrinite-tich coal). The trends for carbon content and atomic H/C are in good agreement with results from North American coals, although the aromaticities of Australian coals obtained in this study appear to be slightly lower than some of those reported for North American coals of similar carbon content.     

Aromaticity of coal extract by 1H and 13C pulsed n.m.r. methods

High-resolution gated decoupling ¹³C spectra of soluble materials such as coal extracts have been analysed to evaluate the carbon aromaticity (f_a). A correlation was found between f_a and the reciprocal of the square of the shorter component in spin—spin relaxation time (T₂), which was obtained by pulsed ¹H n.m.r. in the solid state at low temperature. Values of carbon aromaticity for several coals as received were estimated by using the above correlation and compared with those by van Krevelen's densimetric method.     

Determination of ether oxygen in coals by 13C CP/MAS n.m.r. spectrometry and acetylation

The amount of aliphatic and aromatic ethers in coals with different carbon contents were determined by combined use of ¹³C CP/MAS n.m.r. spectrometry and the acetylation method. The role of etheric oxygen in the depolymerization of coals was re-evaluated. Etheric oxygens were distributed more abundantly in aromatic ether rather than in aliphatic ether and amounted to 3.7% relative to carbon. The cleavage of ether linkages in early stages of depolymerization of coal led to the formation of asphaltene or preasphaltene.     

Structural changes in humic acids during the coalification process

Humic acids from four coals, varying in rank from peat to subbituminous coal, have been characterized by elemental analysis, acidic groups, molecular weight, electrophoresis and visible, FT-i.r. and CP/MAS ¹³C n.m.r. spectroscopy. The humic acids increase in carbon content, molecular weight, condensation degree and aromaticity (f_a) with increasing maturation of the parent coals, while the oxygen content decreases with a loss of oxygen functional groups. The presence of lignin-like polymers, poly-saccharides and peptidic materials in humic acids from peat was established using i.r. and ¹³C n.m.r. spectroscopy. The structural changes observed in humic acids are in agreement with the recognized coalification theory and tend to support the hypothesis of condensation of humic acids into insoluble humin of coal.     

Quantitative solid-state C n.m.r. measurements on cokes, chars and coal tar pitch fractions

Bloch decay or single pulse excitation (SPE) ¹³C n.m.r., generally recognized as the best approach to obtain quantitatively reliable aromaticity values and other skeletal parameters for coals, was applied to partly carbonized coal samples, a biomass char and the toluene-insolubles from a coal tar and a corresponding pitch. As found previously for coals, the aromaticities and non-protonated carbon concentrations were generally higher than those estimated by cross-polarization (CP). Furthermore, in terms of accumulation times, the shorter ¹³C T₁'s of low-temperature chars makes SPE a more efficient technique than for coals. The higher concentrations of paramagnetic centres responsible for the shorter ¹³C T₁'s still result in observability of 75% of the carbon in the chars by the SPE technique. The ratios derived from the SPE measurements agree well with those obtained from elemental analysis.     

New approach to the analysis of the 13C CP/MAS n.m.r. spectrum of coal

A new method of evaluating the ¹³C CP/MAS n.m.r. spectra of coals and related solids has been developed which permits the quantitative assignment of three kinds of aromatic carbons (protonated, substituted and bridge carbons) in addition to alkoxy and aliphatic carbons. The method is based on the analysis of the otherwise troublesome spinning sidebands.     

Structure analysis of coals by resolution enhanced solid state 13C n.m.r. spectroscopy

The application of experimental n.m.r. and chemical resolution enhancement techniques in cross-polarization/magic angle spinning (CP/MAS) ¹³C-n.m.r. spectroscopy yields spectra of coals and coalderived solids which contain structural information within the hybridization resonance envelopes. The sp²- and sp³-carbon resonance manifolds are partitioned into components arising from carbon centres bonded directly to oxygen, hydrogen and only other carbon atoms. The unique, observable chemical shift bands in the spectrum are increased from three (the relative areas of the sp²- and sp³-carbon resonance envelopes and a separate carbonyl band) to nine. This resolution permits the principal structural changes in chemically-modified coals to be mapped in unprecedented detail. The reductive alkylation of a typical bituminous coal has been examined by this method.     

Chemical nature of pre-asphaltenes from flash pyrolysis tars and supercritical gas extracts

Pre-asphaltenes from flash pyrolysis tars of three Australian coals and a supercritical gas extract of one of these coals were studied by ¹³C and ¹H n.m.r. of the silylated pre-asphaltenes. Further information was obtained by hydrogenolysis of one of the pre-asphaltenes followed by g.c. analysis. ¹³C n.m.r. studies of the silylated derivatives and g.c. analysis of the hydrogenated pre-asphaltene showed the presence of long alkyl chains. The aromaticity of the pre-asphaltenes from the flash pyrolysis tars increased as the rank of the coal increased. The pre-asphaltene from the supercritical gas extract was less aromatic than that from the flash pyrolysis tar of the same coal. Average structural data for each of the pre-asphaltenes are reported.     

Spinning sideband suppression and quantitative analysis in solid state 13C n.m.r. of fossil fuels

The use of TOSS spinning sideband suppression to obtain ¹³C n.m.r. spectra of a wide range of aromatic materials is discussed in relation to the spectra of the same materials obtained without sideband suppression. The 39 samples studied include polymers containing both aromatic and aliphatic carbon, asphaltenes, fossil fuels spanning the range in rank from pet to semianthracite, pitch, separated macerals, laboratory oxidized coals and naturally weathered, stockpiled bituminous coals. Despite the diversity in structure, it is found that sideband suppression can be used to obtain apparent aromaticity values consistent with the normal spectra in the majority of samples. The observation of inaccurate aromaticities due to destructive interference between the frequencies associated with magic angle spinning and slow molecular motions in some samples is illustrated.     

Coal flash pyrolysis: 1. An indication of the olefin precursors in coal by CP/MAS 13C n.m.r. spectroscopy

The results reported indicate that the low molecular weight olefins (ethylene, propylene and butadiene) which are major gaseous hydrocarbon products of flash pyrolysis of coal derive from the same precursors in coal, whereas methane, benzene and other pyrolysis products are mainly formed from different components in the coal. CP/MAS ¹³C n.m.r. spectra suggest that the olefin precursors are long-chain polymethylene structures (chemical shift 31 ppm), either chemically bound or mechanically trapped in the coal and thus not solvent-extractable.     

Investigation of the chemical structures of coking and non-coking coals in original and reductively alkylated solid states via 13C n.m.r. and i.r. spectroscopies

Chemical structures of four Turkish coals in original and reductively alkylated forms were investigated in the solid state by CP/MAS ¹³C n.m.r. and i.r. spectroscopies. Dilatation properties of these samples were also determined. It was observed that, while the aliphatic parts of the coking coals are mainly composed of short and straight alkyl groups and alicyclic structures, the aliphatic parts of the non-coking coals are to a larger extent composed of branched chains and/or alicyclic structures. It was concluded that ether bonds linking aromatic units are more prominent in the coking coals, whereas in the non-coking coals aromatic-O-aliphatic and/or alicyclic ethers dominate. The degree of condensation of aromatic structures was found to be higher in the coking coals, and the extent of reductive alkylations was higher in comparison with the non-coking coals. Non-coking coals were found to be alkylated preferentially at their phenolic oxygens. Difference spectra were found to be very useful in following organic chemical structural changes that accompany reductive alkylation.     

Correlation of microautoclave and 1H n.m.r. measurements of coal liquefaction solvent quality

Some 467 coal liquefaction process oils, having nominal boiling points within the range 473–808 K (200–535 °C), were assayed for donor solvent quality in 577 microautoclave tests using three different sets of reaction conditions. Proton distributions were also determined on each sample by ¹H n.m.r. Microautoclave coal conversions and proton distributions, correlated using multiple linear regressions, give similar measurements of solvent quality over a wide range of solvents. Donor solvent quality (as measured by microautoclave coal conversion) increases with increasing hydroaromatic content and with decreasing aromaticity and paraffinicity. As they were designed to do, the different microautoclave tests measure different solvent properties. Above a certain level of solvent quality, microautoclave extractions are insensitive to solvent quality differences. However, ¹H n.m.r. can differentiate between these high quality solvents, and the data can be related to differences in process performance. Advantages of using ¹H n.m.r. for donor solvent quality measurements are discussed.     

Characterization of coals and coal oxidation by magic-angle 13C n.m.r. spectroscopy

Magic-angle ¹³C n.m.r. spectra have been obtained for a series of vitrinite concentrates. Proper modification of the cross-polarization pulse sequence allows separation of protonated and nonprotonated carbon resonances. This technique is used to determine the relative fraction of nonprotonated aromatic carbons for each of the vitrinites, a parameter observed to decrease with increasing rank. Another parameter, related to the aromatic hydrogen content, is also calculated from these data and the results correlate with those from Fourier transform i.r. spectroscopy. The methods used for analysis of the vitrinite concentrates were then applied to the low-temperature oxidation of coal. The fractional aromaticity as determined by n.m.r. increases with longer oxidation times, indicating preferential attack on aliphatic structures. Here the FT-i.r. results are in quantitative agreement with those from n.m.r. Finally, the advantages of using various pulse sequences to extend the range of magic-angle n.m.r. and of combining FT-i.r. and n.m.r. results are discussed in the context of their potential for coal science.     

A novel semi-empirical relationship between aromaticities measured from 1H and 13C n.m.r. spectra

An empirical correlation between ¹H n.m.r. measured aromaticity (H_ar) and ¹³C n.m.r. measured aromaticity (C_ar) has been found. Over 300 samples which are diverse in origins and structural characteristics have been studied. A semi-empirical functional relationship between H_ar and C_ar has been devised. It is suggested that for distillable samples with, nominally, less than 50% boiling above 320 °C, the derived function can be used to rapidly and conveniently approximate C_ar from H_ar. The difference between observed and calculated C_ar values for 303 such samples shows a standard deviation of just 1.8%.     

Study of structural parameters on some petroleum aromatic fractions by 1H n.m.r./i.r. and 13C, 1H n.m.r. spectroscopy

¹H n.m.r. and i.r. spectroscopy were used to derive molecular parameters of petroleum fractions. Relative amounts are estimated of methylene and methyl groups in substituted alkyl side chains bonded to the aromatic ring system. The resolution of equation combinations leads to estimation of $\text{H}{}_{\mathrm{s}}\text{C}{}_{\mathrm{s}}\text{(=x)}$, which is an important parameter for structural analysis. Several petroleum fractions were characterized in terms of hypothetical average molecular structures using ¹H n.m.r./i.r. procedures, ¹³C coupled proton n.m.r. and Brown—Ladner methods. It is proposed that the ¹H n.m.r./i.r. method gives more precise average molecular parameters than the Brown-Ladner method with the most precise analytical procedure, up to date, being ¹³C coupled proton n.m.r. analysis. The Brown-Ladner method is especially suitable for structural analysis of low aromaticity molecular structures with long straight-chain alkyl substituents.     

Cross-polarization n.m.r. as a tool to investigate solvent-coal molecular interactions

The interaction of pyridine and aniline with an Australian bituminous coal (Liddell) has been studied by cross-polarization ¹³C nuclear magnetic resonance spectroscopy (CP ¹³C n.m.r.). The results show that sorbed aniline or pyridine molecules on coal are sufficiently immobilized to cross polarize. Carbon-proton static dipolar interactions, however, are weak which suggests that sorbed molecules can rotate rapidly about one or more of their axes of symmetry. It is also established that spin diffusion between coal and sorbed molecules is inefficient. Some loss of conformational rigidity of coal molecules is indicated when coal is soaked with pyridine.     

Selective saturation as an aid for carbon-13 CP/MAS n.m.r. analysis of coals at high frequencies

A selective saturation experiment is described and applied to selective elimination of ¹³C high resolution solid state n.m.r. signals and of their spinning sidebands, as well for sharp lines as for broad bands. A proper adjustment of experimental conditions permits selective saturation of the aromatic carbon signals of coal samples and eliminates unwanted overlapping of aromatic sidebands with the aliphatic region of the spectrum at high working frequencies. The described procedure seems to be especially useful for qualitative and quantitative high frequency n.m.r. investigations of coals having a distribution of short T₂ relaxation times.     

The characterization of coal tar pitches used in electrode binder manufacture by n.m.r. spectroscopy

Coal tar pitches dissolved in a mixed solvent system comprising S₂Cl₂ and SO₂Cl₂ were analysed by ¹H and ¹³C n.m.r. spectroscopy. The extensive chlorination of the pitch by this solvent unfortunately leads to serious underestimations in aromatic hydrogen concentrations when these solutions are examined by ¹H n.m.r. spectroscopy. However, quantitative data can be obtained using ¹³C n.m.r. spectroscopy where the carbon skeleton is unaffected by the extent of chlorination of the pitch. The ¹H n.m.r. spectra of individual aromatic compounds, similar to those found in coal tars have been recorded. The ¹H n.m.r. spectra confirmed that extensive chlorination of the aromatic nuclei had occurred which results in the non-quantitative estimation of hydrogen concentrations.