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.     

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.     

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 in liquid ammonia

The reductive alkylation of Illinois No. 6 coal was investigated using alkali metals and alkyl halides in liquid ammonia. Potassium is the most effective reducing agent and butyl iodide is the most effective alkylating agent for the preparation of coal alkylate that is soluble in tetrahydrofuran. The overall yield of soluble product is often improved through the reaction of the tetrahydrofuran-insoluble portion of the initial reaction products with an alkylating agent in the presence of tetrabutylammonium hydroxide. The infrared spectra of these materials suggest that the phase transfer agent catalyses the esterification of residual carboxylic acid functions. The intermolecular interactions between such acid groups and acceptor groups markedly restrict the solubility of the coal alkylate. The gel permeation chromatograms of the soluble reaction products are essentially featureless with only modest maxima at short and long elution volumes. The proton and carbon nuclear magnetic resonance spectra of the reductive methylation products, prepared using methyl-¹³C iodide, suggest that carbon alkylation exceeds oxygen alkylation and that the alkylation of phenolic groups is the dominant O-alkylation reaction. The spectra also suggest that fewer ethers are cleaved in the reaction in liquid ammonia than under the conditions of the Sternberg reaction.     

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.     

Structural characterization of non-reductively ethylated coal by 13C and 1H n.m.r.

A major portion of some coals can be solubilized by non-reductive ethylation. A part of the solubilized portion of a bituminous highly caking coal was studied by ¹H and ¹³C nuclear magnetic resonance techniques. It was demonstrated that important structural information can be obtained by this method for the original insoluble material in coal. It was found that, in the investigated coal, about 40% of the aromatic carbon atoms are protonated and that 6–10% of total carbon atoms existed in original coal as active sites that were amenable to ethylation.     

Determination of the apparent ratio of quaternary to tertiary aromatic carbon atoms in an anthracite coal by C-H dipolar dephasing n.m.r.

¹³C-¹H heteronuclear dipolar dephasing n.m.r. techniques allow discrimination between different chemical species contributing to the ¹³C n.m.r. spectra of complex hydrocarbons. Model compound studies show significantly different effective transverse relaxation constants for carboxyl and quaternary carbon atoms (≈200 μs), secondary and tertiary (≈20 μs), and primary carbon atoms (≈80 μs). Use of these effective relaxation data, together with appropriately timed windows in the continuous wave decoupling applied in standard cross-polarization-magic-angle spinning experiments on anthracite coal allow discrimination between aromatic tertiary and aromatic quaternary ring carbon atoms in this coal. Within the accuracy of experimental error, and of the structural modelling experiments herein reported, the use of the dipolar dephasing technique together with results of X-ray diffraction on coals allows a reasonable estimate to be made of the average number of condensed polynuclear rings in an ‘average molecule’ in the anthracite studied. Based on a model of pericondensed aromatic rings, this number lies between 33 and 45.     

Structural studies of coal extracts

The reductive alkylation of a medium-volatile bituminous coal was carried out using potassium and naphthalene in tetrahydrofuran, and using methyl, ethyl and butyl iodides to alkylate the resultant polyanion. The soluble products of the reductive alkylation reaction were isolated by extraction with n-pentane and benzene. The extracts were characterized by i.r. and ¹H n.m.r. spectroscopy, molecular mass and ultimate analyses.     

Evaluation of the aromaticity in French coals by 13C-1H cross polarization,magic angle spinning and dipolar dephasing nuclear magnetic resonance spectroscopy

The aromaticity factor f_a of various French coals, as determined by ¹³C-¹H cross polarization and magic angle spinning, is reported. The f_a, values, which are estimated after a careful examination of the evolution of the ¹³C magnetization as a function of the contact time, are 0.63, 0.74, 0.82 and 0.85, respectively, for the Gardanne, Vouters, Méricourt and Escarpelle coals. The fraction of non-protonated aromatic carbons, fⁿ_a, as determined by dipolar dephasing experiments seems to decrease with increasing rank until a minimum value is reached for the low volatile bituminous coal from Méricourt, followed by an increase for the semi-anthracite coal from Escarpelle.     

Pyridine extracts of solid fuels

Pyridine extracts of a coking coal and of two lignites have been further separated and the ¹H and ¹³C n.m.r. spectra of the various fractions have been recorded. Three methods of separation were investigated: gel-permeation chromatography, successive extraction with petroleum ether and with benzene as used by previous workers for supercritical-gas extracts, and the classical extraction of the fractions. Gel-permeation chromatography gave most promise of effective separation but the fractions obtained often contained too little material to give n.m.r. spectra. The n.m.r. spectra of the chloroform-soluble material from the pyridine extracts of the coking coal were consistent with a material consisting of polynuclear aromatic nuclei joined through simple bridge structures and possibly also through direct carbon—carbon linkages. The nuclei were substituted by branched and unbranched paraffin chains, some of which were 7 or 8 carbon atoms long. It is probable that alicyclic structures were also present. The aromatic nuclei were mobile in solution. 60% of the carbon atoms were aromatic, and it can therefore be calculated that the molecular weight per aromatic nucleus was 200–300. It is suggested that ¹³C n.m.r. spectra may be used to characterize chemicals likely to confer coking properties on solid fuels.     

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.     

An investigation of coal by means of e.s.r., 1H n.m.r., 13C n.m.r. and dynamic nuclear polarization

Sixty coal samples of different rank and origin have been investigated by means of e.s.r., ¹H n.m.r. and ¹³C n.m.r., the last two in combination with dynamic nuclear polarization (DNP). The following parameters have been determined: the number of free radicals, the e.s.r. linewidth, the ¹H Zeeman relaxation rate, the ¹H relaxation rate in the rotating frame, the ¹H DNP enhancement, the ¹³C DNP enhancement, the ¹³C Zeeman relaxation rate and the ¹³C aromaticity, observed via ¹H¹³C cross-polarization (CP), both with and without magic-angle spinning (MAS). The relations between these parameters and coal rank have been investigated. Moreover, with DNP special experiments have been performed which provide information about the localization and the mobility of the unpaired electrons present in these coals. Finally, DNP has been used to investigate various features of the quantitative analysis of coal via ¹³C n.m.r. MAS was found to reduce the measured ¹³C aromaticity, and for three coals it was shown that even without MAS only ≈ 50% of the aromatic ¹³C nuclei are detected by the CP technique.     

Formation and chemical structure of preasphaltenes in short residence time coal hydrogenolysis

The formation and chemical structure of preasphaltenes in short residence time coal hydrogenolysis were investigated. In short residence time coal hydrogenolysis, preasphaltenes are the major product. The maximum yield for this parametric study was obtained under reaction conditions of 500 °C and 21 s. The formation of preasphaltenes reached the maximum value in the initial stage of the liquefaction reaction. As the liquefaction reaction continued, the deoxygenation of preasphaltenes proceeded. However, the decrease in aromatic atoms bound to the hydroxy, methoxy and oxygen atoms of the diphenyl ether group (Arz.sbnd;O) is small, and the ArO functionality still remains abundant in preasphaltenes. Preasphaltene-I is characterized by carbon aromaticity (f_a) of 0.6–0.7, aromatic rings of from 1 to 3–5 per condensed aromatic ring system, 55–70% substitution of aromatic ring carbons and C_2–3 aliphatic substituents. The molecular weight ranges from 500 to 650, and is not much different from that of the asphaltenes. The f_a values based on the Brown-Ladner method and on solid state CP/MAS ¹³C n.m.r. spectra data agree closely.     

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.     

Reactivity of O-methylated Illinois No. 6 coal towards alkyllithiums/methyl iodide

An O-methylated Illinois No. 6 vitrain was treated repeatedly with a series of C-H indicator bases of known pK_a and quenched with ^13,14C-methyl iodide. The extent of ¹⁴CH₃ incorporation depended strongly upon base strength, increasing in the order 9-phenylfluorenyllithium < fluorenyllithium < trityllithium. A significant number of methyl groups ($\text{0.7–1.2}\text{100}$ coal carbons) were introduced with fluorenyllithium and trityllithium as the base. Additional methyl groups were added upon a second and third treatment with base and alkylating agent. After three treatments, the number of added methyl groups had doubled. The extent of base reagent incorporation was established by parallel experiments using ¹⁴C-enriched bases. CP/MAS ¹³C n.m.r. of the first coal alkylation product using trityllithium and ¹³C-enriched methyl iodide was consistent with predominantly C-alkylation. From these results, a unified picture for coal structure emerges which contains a distribution of reactive C-H sites, distinguishable on the basis of their acid-base properties. Possible reactive sub-structures are considered.     

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.     

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.     

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.     

Improved method to alkylate Yūbari coal of Japan using molten potassium under refluxing THF

A modification to Sternberg's procedures of reductive alkylation of coals is proposed. The ‘coal anion’ formation reaction is conducted under refluxing THF without any electron transfer agent and with molten potassium metal. The method was applied to Yūbari coal (86 wt % C) whereby varying reaction times (0.5–6 h) altered the lengths of alkyl groups (CH₃C₄H₉) added. In a typical experiment, a butylated Yubari coal, prepared by the 2 h reaction, contained 7 butyl groups per 100 original carbon atoms and solubility in hot benzene was 75 wt %. The numbers of alkyl groups introduced and the solubilities of coals increased with reaction time. Values obtained were comparable to those reported using conventional procedures which required longer reaction times and an electron transfer agent l.r. spectra of butylated coals showed strong adsorption peaks attributed to aliphatic groups. Benzene-soluble—pentane-insoluble material of the alkylated coal had less condensed aromatic components with more alkyl side-chains compared with Yūbari SRC, which were estimated by the Brown—Ladner method. Contamination of the alkylated coal by THF fragments seems to be negligible, since hydrogenated naphthalene, obtained after treatment with molten potassium, contains no alkylated products. The reaction proceeded also in HMPA, but not in straight-chain hexane.     

Direct preparation of ionic potassium—coal adducts

The possibility for a direct preparation of ionic potassium—coal adduct is shown, from finely ground low-volatile bituminous coal (89.2% C) and metallic potassium with stirring at 120 °C and a vacuum of 13 mPa. The treatment of the adduct dispersed in THF with CH₃l results in reductive methylation of the coal, similar to that of the ‘coal anion’, obtained in THF during its interaction with potassium—naphthalene.