Quantitative FT 13C n.m.r. of solvent-refined coals using sym-triazine as solvent

Aromaticities (f_a) determined for several solvent-refined coals (SRC) by ¹³C n.m.r. in sym-triazine solution and by the Brown and Ladner ¹H n.m.r. technique have been found to be the same within experimental error (0.01). Use of sym-triazine to determine the f_a of the soluble and insoluble fractions of a SRC in several solvents showed that the soluble fraction reflected reasonably accurately the properties of the whole sample for dioxan, carbon disulphide, chloroform and benzene extracts. The properties of the insoluble fraction varied only slightly. A more detailed analysis of the aromatic region of the ¹³C n.m.r. spectrum has led to some useful structural relations.     

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.     

A comparison of the carbon-13 n.m.r. spectra of solid coals and their liquids obtained by catalytic hydrogenation

High-volatile bituminous and lignite coals have been studied by means of carbon-13 n.m.r. employing cross-polarization and magic-angle-spinning techniques. The solids spectra have been compared to those of the coal-derived liquids. By artificially broadening the high-resolution spectra of the liquids, a striking similarity in line shape and position is observed when the liquid spectra are compared with those of the corresponding solid coal. It is noted however that certain bands visible in the solid are no longer present in the liquefied state. The significance of these results is discussed in terms of carbon skeletal structures present in the solids but not in the liquids.     

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.     

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 limitation of 13C CP-MAS n.m.r. spectroscopy for the study of treated coals

Carbon-13 n.m.r. spectroscopy is often considered to be a quantitative method for analysis of coals and treated coals provided the experimental conditions are carefully selected. In some cases, the magnetic impurities strongly affect the spectra leading to non-quantitative measurements and even to the impossibility of observing a spectrum.     

Two-dimensional n.m.r. analysis of aromatic fractions from a coal liquefaction solvent

Two-dimensional n.m.r. experiments correlating heteronuclear ¹H-¹³C and homonuclear ¹H-¹H spin-spin coupling have enabled various aliphatic groups to be identified unambiguously in complex mono- and diaromatic fractions from a coal liquefaction solvent. These groups include those that are generally difficult to identify by g.c.-m.s. such as five membered rings and CH₃ substituted in cyclic structures.     

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.     

Comparison of Australasian tertiary coals based on resolution-enhanced solid-state 13C n.m.r. spectra

¹³C solid-state nuclear magnetic resonance spectroscopy was used to characterize 32 low-rank coals from New Zealand and Austrlia. A combination of high magnetic field (4.7 T) and resolution ennhancement was used to extract spectral details beyond those seen in published spectra of coals of similar rank. Signal heights were used to characterize organic functional distributions. The spectra showed close similarities between Australian brown coals and low-rank New Zealand subbituminous coals, particularly those mined in the North Island. The spectra of New Zealand lignites all showed stronger signals from cellulose, methoxyl groups and phenols. Almost all of the New Zealand coals showed a relatively strong signal from polymethylene chains, compared with the Australian brown coals. This led to a prediction of higher alkene yields from pyrolysis of the New Zealand coals. Variations in phenolic substitution patterns were attributed to variations in the relative proportions of tannins and lignins in the depositional environments.     

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.     

Characterization of organic fractions in solvent-refined coal by quantitative n.m.r. spectroscopy

A solvent-refined coal product obtained from Pittsburgh No. 8 coal has been preparatively separated into four sized fractions by gel permeation chromatography. Quantitative ¹H and ¹³C Fourier-transform nuclear-magnetic-resonance results for the separated fractions are reported along with elemental and molecular weight analysis data. Observed trends for several average molecular parameters for these fractions (e.g. $\text{(}\text{H}\text{C}\text{)}{}_{\mathrm{alp}}$, $\text{(}\text{H}\text{C}\text{)}{}_{\mathrm{aro}}$, etc.) are discussed. The absence of certain organic functional groups, such as carbonyls, is also noted.     

Oil-shale analysis by CP/MAS-13C n.m.r. spectroscopy

Solid-state ¹³C n.m.r. (CP/MAS-¹³C n.m.r.) spectroscopy provides a direct method for estimating potential oil yields of oil shale formations. Relative aliphatic resonance areas correlate linearly with oil yield and provide a method for oil yield estimation that obviates the need to determine weight per cent organic carbon for each specimen. This direct measurement is performed using an internal area standard, the carbonyl resonance of N-(2-¹³C-propanonyl)-N,N,N-trimethylammonium chloride, to monitor spectrometer sensitivity. Oil shale samples obtained as a function of depth at a site in the Mahogany Zone of the Green River Formation show a near-constant aliphatic carbon fraction, f_al ≡ (1?f_a), and a twofold, nonlinear variation in oil yield over the vertical dimension of the sampling. Aliphatic carbon resonance band shape changes among these samples are interpreted qualitatively as reflecting a two component mixture composed of the condensed alicyclic structures which link together to form the kerogen matrix and an n.m.r.-distinct but not necessarily chemically distinct contribution from normal-long chain hydrocarbon residues.     

Investigation of pyridine extracts and residues of coals by solid-state 13C n.m.r. spectroscopy

The ¹³C n.m.r. technique of cross polarization with magic angle spinning was applied to seven coals in their original states and the components obtained after pyridine extraction. It was observed that for the higher rank coals, short and straight alkyl groups are preferentially transferred to the extracts, while long-chain alkyl groups, branched alkyl groups or alicyclic structures are concentrated in the residues. Aliphatic ether or ester moieties were also observed to be concentrated in the residues. For all coals the computer-generated composite extract-residue spectra differed appreciably from the spectra of the original coals, indicating that some structures may be lost or that some structural conversion may occur during the extraction procedure. It was concluded that caution should be used in trying to determine detailed organic structural information of a coal from data obtained on its pyridine extract.     

Coal liquefaction solvents: An n.m.r. analysis of a recycle oil over successive passes

Recycle distillates from a coal liquefaction continuous pilot plant, which used prehydrogenated anthracene oil as a start-up solvent and up to seven successive passes over an operation time of 800 h, were analysed by ¹³C and ¹H n.m.r. to give average structure and molecular parameters. The fraction of donatable hydrogen steadily decreased in the successive recycle solvents, by ≈ 3 wt% over each pass, as a result of a slow degradation of benzonaphthenic rings into saturates and alkyl-substituted aromatics or five-membered hydroaromatics.     

Molecular motion of syndiotactic poly(α-methylstyrene) in solution studied by carbon-13 n.m.r. relaxation measurements

The carbon-13 n.m.r. spin-lattice relaxation times, nuclear Overhauser enhancement factor (NOE), and line widths have been measured for a syndiotactic poly(α-methylstyrene) in solutions in toluene-d₈ and o-dichlorobenzene-d₄ as a function of temperature from 40° to 163δC. The single correlation time model of relaxation is inadequate to explain the data of spin-lattice relaxation time and NOE. But, within experimental error, these relaxation data in two solvents over a full temperature range can be interpreted in terms of either the Cole-Cole or the log-χ² distributions of correlation times, or a conformational jump model proposed by Monnerie et al. The internal rotation of the methyl group is relatively rapid, while that of the phenyl ring is slow and practically overshadowed by the backbone segmental reorientation over the temperature range examined. The solvent dependence of relaxation data was discussed.     

13C n.m.r. study of raw materials for carbon black

¹³C n.m.r. spectroscopy has been applied to investigate a number of coal tar and petroleum-derived carbon black feedstocks. Application of the J-modulated spin echo technique is especially rewarding, as this method renders the quaternary carbons directly detectable. Thus the quality of carbon black feedstocks can additionally be gauged on the basis of the contents of quaternary carbons, which is a direct indication of the carbon yield in the carbon black production process.     

Potential of proton-enhanced 13C n.m.r. for the classification of kerogens

High resolution n.m.r. spectroscopy, involving the technique of cross-polarization, along with magicangle spinning, was used in the structural characterization of eight kerogens of different origins, selected to represent the three types of kerogens (types I–III evolution paths). The influence of cross-polarization dynamics on the sensitivity of the method and the ratio of individual fractions in the spectrum was studied in more detail. It is suggested that an analysis of the influence of the mixing time is necessary prior to definitive characterization of any sample. Good separation of signals in aliphatic, aromatic, and carboxylic regions was achieved. The general correlation between the ¹³C n.m.r. structural characteristics and the classification based on ultimate analysis of the kerogens (types I–III, van Krevelen atomic H/C vs. O/C diagrams) was found to be satisfactory. The structural features of kerogens derived from ¹³C n.m.r. analysis agreed quite well with characteristics constituting the above mentioned classification. The ¹³C n.m.r. method used in this paper may be considered promising in the classification of kerogens.     

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.     

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.     

Estimation of the concentration of donatable hydrogen in a coal solvent by n.m.r.

¹H and ¹³C nuclear magnetic resonance (n.m.r.) spectroscopies have been used to study the changes in composition of a hydrogenated anthracene oil solvent that occur during extraction of a UK bituminous coal. It has been shown that the principal hydrogen donor groups in the solvent are hydroaromatic and naphthenic rings in hydroaromatic species. Good agreement was achieved between the concentration of donatable hydrogen estimated from n.m.r., and the amount of hydrogen transferred from the solvent during exhaustive extraction.