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.     

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.     

13C n.m.r. chemical shielding tensor elements of solid poly(methyl methacrylate)

The solid-state non-spinning ¹³C nuclear magnetic resonance spectrum of poly(methyl methacrylate) (PMMA) is reassigned. Principal elements of the chemical shielding tensors are obtained for all carbon resonances. The observed anisotropies indicate that the polymer, both with respect to the side chains and the backbone, is essentially rigid at room temperature on the time scale of the experiment (in the millisecond range).     

High resolution 13C n.m.r. spectra of solid isotactic polypropylene

The high-resolution ¹³C n.m.r. spectra of three samples of solid isotactic polypropylene are reported. The spectra, obtained under conditions of proton dipolar-decoupling and fast magic-angle rotation and using cross-polarization, are of annealed and quenched samples of the α-crystalline form and of a sample of the β-crystalline form. Attention is drawn to the importance of knowing the proton relaxation characteristics in these experiments and some illustrative proton T_1? data are given. The ¹³C n.m.r. spectrum of the annealed sample of the α-crystalline form shows well-resolved splittings of the methyl and methylene resonances in a 2:1 intensity ratio. These splittings are interpreted in terms of the crystal structure of the α-form as suggested by X-ray diffraction. Quenching the α-form causes significant changes in the spectrum including a loss of resolution of the splittings obtained from the annealed sample. The β-form shows broad symmetrical resonances for the methyl and methylene carbons. The chemical shifts and other spectral features are discussed in the light of the proposed crystal structures and the effects likely to be produced by quenching.     

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.     

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.     

High resolution solid state 13C n.m.r. of butadiene grafted polypropylene

¹H dipolar decoupled ¹³C n.m.r. experiments with the aid of the magic angle spinning technique have been used to determine the graft content and the isomer content of the polybutadiene grafts in a graft copolymer with isotactic polypropylene.     

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.     

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.     

Evaluation of solvents for carbon-13 n.m.r. analysis of solvent-refined coals. sym-Triazine a new carbon-13 n.m.r. solvent

Solvent-refined lignite (SRL) and bituminous coals (SRC) are only partially soluble in good standard carbon-13 n.m.r. solvents. A survey of other possible solvents for SRLs and SRCs showed that sym-triazine at 86 °C solvated SRL as well as pyridine, but did not absorb in the regions of interest (5–60 and 90–160 ppm relative to TMS). Model hydrocarbons, phenols and an alcohol in sym-triazine gave aromaticities (f_a) accurate within the standard deviation (0.01 to 0.001) when proper FT carbon-13 techniques were used. An alkene and an aliphatic amine reacted with sym-triazine and gave inaccurate values of f_a.     

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.     

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.     

A 13C n.m.r. study of the sulphonation of novolacs

A parameter scheme has been devised for predicting the ¹³C shifts of atoms in the aromatic rings of phenols substituted with SO^?₃, CH₂Ar and CH₃ in an alkaline medium, from the measured shifts of 30 different substituted benzene rings. The application of the scheme to novolacs is considered before the scheme is used to interpret the changes of chemical shift that occur upon sulphonation of one such sample. In the aromatic carbon region the sites of substitution on the rings are identified. In the upfield region of the spectrum a substantial alteration to the pattern of shifts from the bridging methylene groups was noted: once a ring had been sulphonated the methylene bridge might break to yield methylol groups, which then to a large extent reformed as methylene and dimethylene ether (CH₂OCH₂) links, but with an increased proportion of abutments on sites para with respect to the phenolic group.     

High resolution solid state C n.m.r. of Canadian peats

Peat samples from three locations in Quebec, Canada, were characterized by ¹³C CP/MAS n.m.r. spectroscopy. The n.m.r. analysis indicates that the peats contain a significant amount of unaltered plant components including cellulose, hemicellulose, lignin, waxes and resins. Integration of specific regions of the spectra can give semi-quantitative estimates of some of these plant components and, in limited cases, information about the degree of decomposition can be obtained from the ¹³C CP/MAS n.m.r. spectrum. Dipolar dephasing experiments and difference spectroscopy were found to be valuable techniques for spectra assignments and enhancement of compositional differences.     

Characterization of polyurethane elastomers by 13C n.m.r. spectroscopy

A ¹³C n.m.r. study of a series of polyurethanes and selected model compounds is reported. Comparison of the observed chemical shifts with values predicted by the Grant and Paul additive theory shows reasonable agreement. The latter approach, therefore, is a useful method for the assignment of carbon resonances in these systems. The positions of carbonyl peaks are more difficult to predict, but model compound studies allow their unambiguous assignment. These peak positions are very useful for qualitative and quantitative measurements. Nuclear Overhauser Enhancement (NOE) factors have been measured for typical copolymers, and it is possible to obtain quantitative estimates of the compositions of unknown polymers and prepolymers.     

Evaluation of a delayed coking process by H and C n.m.r. spectroscopy: 2. Detailed interpretation of liquid n.m.r. spectra

The ¹H and ¹³C nuclear magnetic resonance (n.m.r.) spectra of a delayed coking feedstock and products, presented in a previous paper, are analysed here in detail by matching results from inspection of both nuclei. The conventional spectra of whole samples are used in combination with elemental analysis data, and substantial agreement is obtained. Differences are explored to yield valuable information. Interpretation of some band assignments is modified to account for new results. This approach results in a detailed quantitative estimation of a few key structures that contain the main functional groups that characterize these petroleum fractions.