Estimation of aliphatic H/C ratios for coal liquefaction products by spin-echo 13C n.m.r.

The direct estimation of aliphatic H/C ratios for coal liquefaction products using a part-coupled spinecho (PCSE) ¹³C n.m.r. method is described. The method, which enables proportions of aliphatic C, CH, CH₂ and CH₃ groups to be deduced, has been applied to a low molecular weight (<200) aromatic fraction of hydrogenated anthracene oil (HAO) and to higher molecular weight (?280) aromatic and asphaltene fractions of supercritical gas (SCG) extracts and a hydrogenation residue (pitch). Aliphatic H/C ratios for these occur in the range 1.9-2.3, the SCG extract fractions having the highest values because they contain significantly more CH₃ and less CH than the HAO and pitch fractions.     

Wide-line H n.m.r. study of perdeuterated pyridine imbibed in a premium coal

The motion of perdeuterated pyridine imbibed in Argonne premium high-volatile coal (no. 601) has been studied by wide-line ²H n.m.r. spectroscopy. The coal sample had been saturated with perdeuterated pyridine for several days, then evacuated until the pyridine reached a concentration of 3.5%. The entrapped pyridine molecules undergo two tyes of rotation about the twofold symmetry axis: rapid diffusional rotation with a correlation time of 1 × 10⁻⁷ s; and 180 ° jumps with a residence time of 6 × 10⁻⁶ s. It is estimated that roughly half of the pyridine molecules execute each kind of motion. There is no evidence for a pyridine fraction that executes rapid isotropic rotation. The anisotropic nature of the rotation indicates that the interaction between pyridine and the coal structure is strong enough to prevent isotropic rotation.     

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.     

1H n.m.r. relaxation in bituminous coal and semicoke

Temperature and frequency-dependent proton relaxation measurements, in conjunction with spin concentrations determined by e.s.r., have shown that for fresh and oxidized coal the relaxation mechanism involves spin-diffusion to paramagnetic centres and to rotating methyl groups on alkyl chains. ¹H relaxation in semicokes is more complex; probably both direct relaxation to paramagnetic centres as well as spin-diffusion contribute.     

The application of the f.m.r. technique to coal characterization studies

The mineral magnetite has been detected in a sample of Pittsburgh No. 8 coal by the ferromagnetic resonance technique. The magnetite particles contained in a ground sample were < 0.07 μm diameter and have a chemical composition between Fe_2.98804O₄ and Fe_2.99704O₄. The results of acid treatment indicated that the magnetite is not embedded in the organic coal material, but is readily removed. The use of the f.m.r. technique as exemplified in the present work has important potential in the study of ferro-and ferrimagnetic minerals in raw coal and their interactions with paramagnetic organic free radicals during coal conversion processes.     

Two-dimensional n.m.r. characterization of cycloaliphatic epoxy resins

The cycloaliphatic epoxy resins were studied by various two-dimensional nuclear magnetic resonance (2D n.m.r.) spectroscopies. The ¹³C n.m.r. spectra of the epoxy resins were assigned by using DEPT and 2D INADEQUATE techniques and the ¹H n.m.r. spectra were assigned by using 2D ¹H---¹H and ¹H---¹³C COSY techniques. Complete characterization of the samples synthesized by the oxidation of cyclohexene derivatives not only revealed the existence of the stereochemical isomers resulting from the structural difference of the oxide rings in the samples but also provided the composition ratio of the isomers.     

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.     

Investigation of aromatic carbon sites in materials derived from petroleum and coal using 13C n.m.r. methods

Aromatic C and CH carbon sites in a variety of petroleum and coal derived materials have been investigated using a ¹³C n.m.r. technique termed spin echo broad band off-resonance decoupling (SEBBORD). Only resonances due to non-protonated aromatic carbon sites are observed in SEBBORD spectra such that comparison with conventional ¹³C n.m.r. spectra enables differentiation between aromatic C and CH group resonances. Relative abundances of non-protonated aromatic carbon sites calculated from SEBBORD data are in good agreement with values derived from a combination of conventional¹H n.m.r., ¹³C n.m.r. and elemental analysis data. The occurrence of significant proportions of aromatic C intensity to high field of 129–130 ppm and of aromatic CH intensity to low field of 129–130 ppm has been found to be quite common. Consequently attempts to determine aromatic C and CH group abundances by partitioning conventional ¹³C n.m.r. spectra in the vicinity of 129–130 ppm can lead to considerable quantitative errors. SEBBORD provides more detailed information about aromatic carbon sites than can be obtained from conventional ¹³C n.m.r. spectra.     

Interaction between coal and liquefaction catalysts studied by high temperature electron spin resonance

The high temperature electron spin resonance technique has been used to obtain in situ information on the behaviour of liquefaction catalysts during coal pyrolysis. The spin concentration in coal was induced in the presence of a catalyst at the pyrolysis temperature. ZnCl₂ drastically increased the spin concentration of coal. The order of activity of the catalysts with respect to the increase in spin concentration was: ZnCl₂ (impregnated) ?ZnCl₂ (dispersed) >ZnCl₂/KCl>SnCl₂ > SbCl₃≈AlCl₃ ≈CaCl₂ > coal alone.     

Application of high-field,high-resolution 13C CP/MAS n.m.r. spectroscopy to the structural analysis of Yallourn coal

High-field, high-resolution ¹³C cross-polarization and magic angle spinning nuclear magnetic resonance experiments are reported on Yallourn brown coal and on products obtained from the coal by heat treatment. The spectral resolution at 75 MHz is much improved over that previously reported for lower frequencies. A comparison of experimental and simulated spectra allowed the distribution of carbon types in Yallourn coal to be determined; the carbon aromaticity is 42%, and 30% of the carbon is present in methylene groups. The method was also used to determine structural changes during pyrolysis. At 773 K, aliphatic carbon structures are lost, but higher temperatures are necessary to decompose phenolic structures.     

Optimum conditions for high resolution C n.m.r. of coal- and oil-derived products

Factors affecting the spin-lattice relaxation times T₁ of ¹³C resonances at 62.9 MHz were investigated for simple mixtures of tetralin, ethylbenzene, pentane and phenol and the prominent resonances of an anthracene oil and a coal-derived hydrogenation oil. Parameters varied were solvent temperature and nature—Cr(acac)₃, Cr(dpm)₃, Cr(hfac)₃, Fe(acac)₃, Fe(dpm)₃—and concentration of a paramagnetic relaxation agent (PARR). Effects of PARRs on relaxation rates are strongly dependent on neighbouring acidic or basic centres, but are much more uniform for Cr(hfac)₃ than for Cr(acac)₃. The presence of basic or acidic compounds does not significantly affect the relaxation of ¹³C of the neutral components. Optimum experimental conditions are characterized by nearly uniform T₁ values of all resonances to be detected. While 2D spectra allow disentanglement of overlapping lines, choosing optimum conditions for the neutral constituents may lead to a loss of signals from the acidic or basic compounds.     

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.     

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.     

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.     

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

¹H 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-d₅ and D₂SO₄ 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.     

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.     

C cross-polarization n.m.r. examination of the Glen Davis oil shale deposit

The use of solid-state ¹³C cross-polarization n.m.r. for rapid analysis of bore cores is demonstrated by examination of subsections of a Glen Davis oil shale bore. The determined aromaticities, f_a, differ widely between subsections indicating large variability in potential oil yields. The variation does not correlate with depth of subsection but is shown to correlate well with maceral composition. The correlation found previously by Zilm and co-workers (Fuel 1981, 60, 717) between f_a, determined by solid-state ¹³C n.m.r., and wt% fixed carbon for a range of macerals is found to be supported by the data for Glen Davis subsections which are mixtures of all three maceral types.     

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.     

Evaluation of the aromatic structure of coal tar pitch by solid and solution state n.m.r.

Rapid magic angle spinning has been used to obtain sideband-free high field ¹³C n.m.r. spectra of coal tar pitches in the solid state. However, the unfavourable cross-polarization and ¹H relaxation characteristics make it difficult to accurately estimate concentrations of quaternary aromatic carbon using the dipolar dephasing method. The concentrations of bridgehead aromatic carbon in the whole pitch and toluene-insoluble fraction investigated were deduced using a combination of solid and solution state n.m.r. and elemental analyses.     

Two-dimensional n.m.r. studies of polybutadiene adducts with dihalocarbenes

A variety of two-dimensional (2D) n.m.r. experiments were performed on the adducts of 1,4-polybutadienes (PBD) with difluorocarbene (:CF₂) and fluorochlorocarbene (:CFC1) to assign stereochemical microstructure, carbon types, and syn and anti isomers. A 2D homonuclear ¹⁹F J-correlated (COSY) spectrum of the trans-PBD:CF₂ adducts at complete conversion showed that the geminal fluorines were equivalent in the isotactic and syndiotactic stereochemical triads (AA' singlets), but nonequivalent in the heterotactic triad (AB quartet). The 2D homonuclear ¹⁹F J-resolved experiment confirmed this finding and gave a geminal coupling constant of 160 Hz. Pentad stereosequence fine structure was observed in the 2D spectra but not in the 1D spectra. Editing of carbon types proved difficult by the distortionless enhancement by polarization transfer (d.e.p.t.) pulse sequence (Doddrell, D. M., Pegg, D. T. & Bendall, M. R., J. Magn. Reson., 1982, 48 , 323) for PBD:CFC1 adducts, owing in part to different magnitudes of the ¹J₁₃ scalar coupling constant. A 2D heteronuclear ¹³C-¹H J-resolved experiment was superior and also provided the coupling constants, which ranged from 130 Hz (acyclic methylenes) to 167 Hz (cyclopropyl methines). The identity of syn and anti structures in cis-PBD:CFC1 adducts was rigorously established by a 2D heterocorrelated ¹⁹F-¹H nuclear Overhauser experiment (NOESY). It is noteworthy that the more hindered structure with chlorine syn to the alkyl groups is preferred by 1·65:1. Analysis of a model compound obtained from cis-5-decene gave ³J couplings of 20 and ≤ 3 Hz for the syn and anti compounds, respectively. These couplings were obscured in the 1D polymer spectrum by stereosequence broadening, but their values could be extracted by 2D ¹⁹F-¹H J-resolved spectroscopy.