Nuclear magnetic resonance spectra of two reductively ethylated fuels

A lignite (67.2% C daf) and a coal (89.0% C daf) have been reductively ethylated by Sternberg's procedure⁴ and the spectra, in particular the ¹H and ¹³C n.m.r. spectra, of the products have been obtained. The distribution of hydrogen atoms and, in less detail, of carbon atoms in the reductively ethylated fuels has been determined. It is emphasized that both reductively ethylated fuels contained a variety of structures. The carbon aromaticities of the reductively ethylated lignite and the reductively ethylated coal were 0.41 ± 0.08 and 0.40 ? 0.54 respectively. The aromatic nuclei of the reductively ethylated coal were more highly substituted than those of the reductively ethylated lignite. The ¹H n.m.r. spectra indicate that the reductively ethylated coal and lignite possessed ≈3.5 and ≈2.5 substituents per benzene ring respectively. Resolved absorption in the ¹³C n.m.r. spectra suggests lower figures than these and it was evident that some of the benzene rings in both materials but especially in the reductively ethylated lignite were lightly substituted. The ¹³C n.m.r. spectra show that both materials contained branched and unbranched paraffin chains. Much of the ¹³C n.m.r. spectra however consisted of unresolved absorptions and the ¹³C n.m.r. measurements suggest that the reductively ethylated materials consisted of rigid, relatively immobile molecules of moderate molecular weight. ¹³C n.m.r. absorption by carbon atoms adjacent to aromatic rings was amongst that which was not resolved — though the rings themselves gave resolved absorptions — and this suggests that the aromatic rings were on the periphery of a rigid, perhaps alicyclic, structure.     

Depolymerization of Turkish lignites. 2. Effect of phenol concentration in a closed system

A lignite (C, 66.9 wt%) was depolymerized, using sulphuric acid as a catalyst, in a closed system in which the phenol/coal ratio was varied from 1.5 to 10. The products were separated by solvent extraction and silica gel chromatography. The i.r. and n.m.r. spectra, and the molecular weight of the products were measured. In the experiments with a phenol/coal ration of 10, complete depolymerization of the coal was seen provided the temperature of depolymerization was at least 210 °C. The products generally contained disubstituted aromatic structures connected by methylene bridges, it was found that as the phenol/coal ratio was increased there was a decrease in the number of methylene bridges connecting the aromatic structures. The molecular weights of the straight-chain pentane and benzene-soluble material were lower than the molecular weights of similar fractions in depolymerization experiments carried out in open systems. A method for the structural analysis of straight-chain pentane and benzene-soluble material based on i.r., ¹H n.m.r. and molecular weight measurements is suggested.     

Average structure of extracts of a butylated coking coal

A high-vitrinite coking coal (87%C, daf) was subjected to reductive and non-reductive butylation and the yields of their chloroform extracts (butylated coal basis) were 67.2 and 17.1 wt% respectively. The extracts were separated by adsorption and gel permeation chromatography into a few fractions so that the latter were in ‘equimolar’ ratios (considering the average molecular masses). The soluble fractions were characterized by elemental and average molecular mass analysis and data derived from ¹H and ¹³C n.m.r. spectra and average structural models of the fractions were constructed.     

Reductive alkylation of illinois No. 6 coal. 1H and 13C n.m.r. spectra of the 13C-enriched alkylation products

The reductive alkylation of Illinois No. 6 coal has been carried out using potassium and naphthalene in tetrahydrofuran and methyl-¹³C and butyl-1 -¹³C iodides to alkylate the resultant polyanion. The soluble products of the reductive alkylation reaction were isolated by extraction and chromatography. Proton and carbon n.m.r. spectra were recorded. The intense resonance signal at δ3.95 which appears in the proton n.m.r. spectra of Illinois No. 6 coal butylated with unenriched butyl iodide is split into a doublet by the ¹³C nuclei. Similar results were obtained for the methylation products. The chemical shift and coupling interaction establish that aryl ethers are a very important constituent of the alkylated coal. The carbon n.m.r. spectra of the coal alkylated with ¹³C-enriched alkyl iodides are intense. The resonances of the C-alkylation products appear in a single broad band with a maximum intensity in spectral regions compatible with the formation of the reductive alkylation products of certain polynuclear aromatic hydrocarbons or the base-catalysed alkylation of certain benzylic carbon atoms. The resonances of the N -alkylation products appear in two distinct bands. These resonances are tentatively assigned to amines produced as a result of reductive alkylation of heterocyclic compounds. The resonances of the 0-alkylation products appear in three distinct bands which can be assigned to alkyl aryl ethers, alkyl aryl ethers with substituents at the adjacent positions, and to alkyl carboxylates. The ratio of ethers to carboxylates in the soluble alkylation products was determined to be 7.8 for butylation and 8.0 for methylation. The Chromatographic fractions contain different amounts of C-, N-, and 0-alkylation products. This finding suggests that the coal structure is not highly uniform.     

Characterization of high-boiling petroleum distillate fractions by proton and 13C nuclear magnetic resonance spectrometry

High-boiling (535–675 °C) distillate fractions of Wilmington (Calif. USA) and Gach Saran (Iran) crude oils were separated into saturate, monoaromatic, diaromatic and polyaromatic-polar fractions by passage through a silica-gel—alumina dual packed chromatography column. These fractions were further separated on the basis of molecular volume by gel-permeation chromatography (g.p.c.). Select g.p.c. fractions were then analyzed by ¹H and ¹³C n.m.r. spectroscopy. The fraction of aromatic carbons (Ar-C) of the total carbon content of a given series of g.p.c. fractions, obtained directly from the ¹³C n.m.r. spectra, showed a significant range of values (e.g. 13.2% to 29.2%) within each series. Furthermore, the values of %Ar-C within a series of g.p.c. fractions showed a maximum in each case. No such maximum was observed in the ¹H n.m.r. spectra for the fraction of aromatic proton (%Ar-H) content compared to the total proton content of any of the g.p.c. fraction series. Signals observed in the ¹³C n.m.r. spectra confirmed the presence of aliphatic chains attached to aromatic structures in the monoaromatic, diaromatic and polyaromatic-polar g.p.c. series of each distillate fraction. Well-defined signals, not attributable to straight-chain aliphatic material, were also observed in the ¹³C n.m.r. spectra. Comparison of the chemical shifts of these ¹³C n.m.r. signals with those spectra of model compounds obtained experimentally, and with appropriate systems in the literature, strongly suggested the presence of saturated terpenoid-like structures as well as the presence of methyl groups in sterically hindered positions on an aromatic ring system. Gated decoupling techniques and the use of a relaxation agent were used to overcome the deleterious effects of slow relaxation times and Nuclear Overhauser Enhancement (NOE) on the analytical quality of the ¹³C n.m.r. spectra.     

Aromatic products of 340 °C supercritical-toluene extraction of two Turkish lignites: an n.m.r. study

Fractions of Elbistan and Seyitomer (Turkish) lignites, extracted with supercritical toluene at 340 °C and 8 MPa, have been separated by solvent extraction and silica-gel chromatography. Analyses by n.m.r. and i.r. spectroscopies and other methods have been combined in structural-analysis schemes to yield information about the average molecule in aromatic extracts. Carbon aromaticities, f_a, derived from 22.63 MHz ¹H-decoupled pulse Fourier-transform (PFT) ¹³C-n.m.r. are more widely spread for Elbistan (0.34–0.56) than for Seyitomer (0.40–0.43), and are lower than for supercritical-gas (SCG) products from bituminous coals. ¹³C-n.m.r. also reveals the presence of aromatic ether-O in polar fractions. Narrow aromatic signals in 100 MHz ¹H-n.m.r. spectra suggest the presence of single-aromatic-ring average structures. In the hexane-soluble aromatics, 27% (Elbistan) and 29% (Seyitomer) of the available sites are substituted by alkyI groups, some of which are at least eight carbon atoms long; the hexane-soluble polar and asphaltene/asphaltol fractions contain fewer such groups.     

Solid state magnetic resonance spectra of Illinois No. 6 coal and some reductive alkylation products

Illinois No. 6 coal and its reductive methylation and butylation products have been studied by magnetic resonance techniques. Conventional CP-MAS¹³Cn.m.r. spectroscopy indicates that 62% of the carbon atoms in the coal are aromatic and that about 6% of the carbon atoms are carbonyl. Esters are more abundant than carboxylic acids. The resonances of methoxy groups and other novel etheric carbon atoms are apparent in the high field region. Dipolar dephasing experiments suggest that methyl carbon atoms constitute no more than 16% of the carbon, methylene and methine carbon atoms about 14% and quaternary aliphatic carbon atoms about 2%. The dipolar dephasing experiments in conjunction with previous work also permit estimates of the hydrogen atom distribution. The THF-insoluble products obtained in the reductive alkylation reactions with¹³C-enriched alkylating agents contain paramagnetic and ferromagnetic substances that adversely influence the solid state n.m.r. spectra. However, good ¹³C n.m.r. spectra were obtained after these substances were extracted with aqueous hydrochloric acid. The O:C alkylation ratios are 1.2 and 1.0 for methylation and butylation, respectively. Dipolar dephasing experiments establish that the decay constants of functional groups in the whole coal and of C- and O-methyl-¹³C and C- and O-butyl-1 -¹³C nuclei in the labelled coal molecules are very similar to those of reference compounds. These findings suggest that the decay constants measured for the ¹³C nuclei in coals and coal-derived solids provide reliable information about their degree of substitution.     

Mild oxidation of reductively ethylated solid fuels

A reductively ethylated lignite and a reductively ethylated coking coal have been oxidized by alkaline nitrobenzene. The products of the oxidation were separated into fractions by extraction with organic solvents, and the compounds present in the resulting solutions were characterized by a combination of gas chromatography and mass spectrometry. These compounds had molecular weights less than 300 and the oxidation, though mild, had fragmented the reductively ethylated fuels. Many problems remain, but the method appears to lead to an understanding of the structures of reductively ethylated solid fuels. The structures of the oxidation products are listed and discussed; in general they confirm the interpretation of the n.m.r. spectra of reductively ethylated solid fuels².     

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.     

Characterization of coal liquids by 13C n.m.r. and FT-i.r. spectroscopy — fractions of oils of SRC-I and asphaltenes and preasphaltenes of SRC-I and SRC-II

Coal-derived products of SRC-I liquefaction of Blacksville coal and vacuum tower bottoms (VTB) of SRC-II liquefaction of Powhatan mine coal (both bituminous and from the Pittsburgh Seam) were separated into fractions by solvent extraction. The SRC-I was first extracted with ethylacetate, and solubles were subsequently separated into oils and a mixture of asphaltenes and preasphaltenes (APA). The VTB was Soxhlet extracted with pentane to remove any residual oils, followed by tetrahydrofuran to recover APA. The APA portions were then separated by sequential elution solvent chromatography (SESC) into fractions differing in chemical functionality, and then examined by ¹³C n.m.r. and FT-i.r. spectroscopic techniques. APA are intermediates and end products of coal liquefaction. SRC-II APA are of higher molecular weight than the APA of SRC-I. The lower numbered fractions of APA of SRC-I in SESC separation have the same functional groups as the corresponding fractions of middle and heavy distillates. However, the higher numbered fractions are rich in oxygen, which is mainly in carbonyl groups. Part of the carbonyl groups are in esters which cross-link aromatic clusters. Therefore, APA and the coal itself are ‘oligomeric’ in structure, with aromatic clusters linked by carbon bridges with different functional groups. The nature of carbonyl groups in APA has been analysed in detail.     

Hydrogenolysis products of pyridine extract of a Japanese lignite

Petroleum ether insoluble fractions of a pyridine extract of a Japanese lignite were pyrolysed with tetralin. The chain length of alkyl groups bonded to aromatic rings and the average chemical structure of aromatic compounds in the lignite were obtained from analyses of hydrogenolysis products. Gas Chromatographic analyses of fractions eluted by cyclohexane showed the presence of C₁₅ to C₃₄ straight-chain alkanes. The chemical structures of the other fractions were analysed quantitatively by a combination of ¹³C and ¹H n.m.r. The unit structure of petroleum ether insoluble fractions in the lignite consists of one to three aromatic rings with straight-chain alkyl groups, some of which are about 25–30 carbon atoms long.     

Separation and spectroscopic analysis of the aromatic fraction of oils from lignite depolymerization

The aromatic fraction of oils obtained by catalytic depolymerization of a Spanish subbituminous coal has been separated by means of liquid chromatography on partially deactivated alumina into six subfractions, which were studied by i.r. and ¹H n.m.r. spectroscopy. The results indicate hydroaromatic structures with a low degree of condensation, with one to three aromatic nuclei on average, short chain alkyl substituents and significant amounts of -OH and aromatic ether groups.     

Structural characterization of coal-hydrogenation products by proton and carbon-13 nuclear magnetic resonance

Heavy oil derived from coal hydrogenation was separated into saturated fractions, neutral aromatic oil, and asphaltene, and these materials were subsequently fractionated according to the magnitude of their respective molecular sizes by gel-permeation chromatography. These GPC subfractions were analysed by proton and carbon-13 n.m.r. spectroscopy and by an additional procedure using gas chromatography for the paraffinic GPC subfractions. ¹³C-n.m.r. spectra for the GPC subfraction of saturated material showed typical long straight-chain paraffin spectral patterns accompanied by iso-and cycloparaffinic carbon signals. The results from gas-chromatographic measurement for the paraffinic GPC subfractions agree fairly well with the trends of average carbon numbers and contents of straight-chain paraffins obtained by varying the fraction numbers, estimated from ¹³C-n.m.r. analyses. The ratios of aromatic carbon to total carbon (f_a) for aromatic oil and asphaltene GPC subfractions obtained directly from ¹³C-n.m.r. spectra are slightly lower than the results from the ¹H-n.m.r. method assuming x = y = 2 in the Brown—Ladner equation. Peak intensities of the respective carbon species in ¹³C-n.m.r. spectra were compared with the peak intensities of correspondingly bonded species obtained from ¹H-n.m.r. measurement. Some inadequacy was recognized in both measurements. It is assumed that there are two reasons for the discrepancy, one of which is the inaccuracy of ¹³C-n.m.r. results owing to the long relaxation times and the effect of Nuclear Overhauser Enhancement, and another is the application of unsuitable values of x and y for calculations from the Brown—Ladner equation. New analytical treatments for ¹³C-n.m.r. results in combination with ¹H-n.m.r. analyses are suggested in this study to avoid these uncertainties in structural analyses. From this procedure, it is believed that the actual contents of aromatic and aliphatic carbon and appropriate values of x and y can be derived.     

Liquefaction reaction of coal: 2. Structural correlation between coal and its liquefaction products

The structural correlation between coal and its liquefaction products has been examined using cross-polarization, magic angle spinning (CP/MAS) ¹³C n.m.r. and field ionization mass spectrometry (f.i.m.s.). The CH₂/aromatic carbon ratios of all solid products (asphaltene, preasphaltene and residue) were close to the corrected +CH₂/aromatic carbon ratio for the coal. This suggests that the ring structure of the structural unit of each solid product is essentially similar to that of the parent coal, except for a difference in the degree of polymerization of the structural units. The CH₂/aromatic carbon ratios of aromatic ring-type oil fractions also correlated with the corrected ratio for the coal, although they were larger. The z series distribution obtained from the f.i.m.s. of oil fractions revealed that coal with a higher CH₂/aromatic carbon ratio produced an oil rich in naphthenic structures.     

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.     

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.     

Structural analysis of supercritical-gas extracts of coals

The chemical structure of two extracts prepared by supercritical extraction of low-rank coals with toluene, with and without hydrogen, have been determined using solvent and Chromatographic fractionation followed by ultimate analysis, ¹H nuclear magnetic resonance (n.m.r.) spectroscopy, molecular weight and OH measurements. ¹³C n.m.r. and i.r. spectroscopy have been used to obtain confirmatory evidence. The extract obtained in the absence of hydrogen, which amounted to 27% of the coal, was found to contain aromatic structures linked by ring-joining methylene or heterocyclic groups. The hydrogen-assisted extract (47.5 wt % coal) was more condensed and contained more smaller molecules, apparently as a result of the cleavage of heterocyclic groups in the coal. It also incorporated more hydroaromatic substituents. ¹³C n.m.r. showed that virtually all the non-phenolic oxygen was present as aromatic ether.     

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.     

Average structures of petroleum pitch fractions by n.m.r. spectroscopy

Ashland A-240 pitch was fractionated by sequential solvent extraction and hypothetical average molecular structures for the whole pitch and its fractions were determined by using ¹H and ¹³C n.m.r. spectroscopy. The average molecules contain condensed aromatic ring systems substituted by short alkyl side chains in both A-240 pitch and the pitch fractions. This result conflicts with Seshadri et al's claim that the Ashland pitch fractions consist of molecules with uncondensed aromatic ring systems linked by aliphatic bridges. It is shown that Seshadri et al's results are probably based on nonquantitative ¹³C n.m.r. data due to the use of a low concentration of paramagnetic relaxation agent.     

Proton and carbon n.m.r. spectra of butylated coal

The proton and carbon n.m.r. spectra of butylated Illinois No. 6 coal which has been fractionated by gel-permeation chromatography (GPC) are reported. The chemical shifts and spin-lattice relaxation times of model compounds provide the bases for the assignment of the spectroscopic results. The spectra of the various molecular weight fractions are clearly different. There are significant variations in the degree of aromaticity, the presence of linear alkanes, the ratio of C-butylation to O-butylation, the extent of butylation on aliphatic and aromatic carbon atoms, and the amount of carbonyl and vinyl derivatives. The results suggest that electron transfer, proton abstraction, ether cleavage, and elimination reactions are important under the conditions of the Sternberg process and that both nucleophilic and free-radical substitution reactions occur. The results also strongly suggest that the distribution of functional groups in coal is not uniform.