Estimation of coal aromaticities by proton-decoupled carbon-13 magnetic resonance spectra of whole coals

A recently developed nuclear magnetic resonance (n.m.r.) cross-polarization technique has been applied to obtain high-resolution carbon-13 spectra of vitrains from two coals. The method, which overcomes the problem of dipolar line broadening by protons, permits estimates of the carbon aromaticities of solid coals. Preliminary results support the classical views that coals are highly aromatic materials and that the aromaticity of coal increases with increasing rank. Limitations of the technique, the accuracy of representation of the distribution of carbons in the sample, and consequently the reliability of the resulting f_a values are discussed.     

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.     

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.     

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.     

Activity of coals from mongolian deposits in the process of thermochemical degradation

The composition and technical characteristics of a series of coal samples from the deposits of Mongolia were determined. Reactivity in the process of thermochemical degradation in tetralin with obtaining liquid products was studied. Based on the results of comparative tests, it was found that the brown coals of Mongolia from the Baganuur, Tevshiin Gobi, and Khovil deposits are of the greatest interest for destructive processing into liquid hydrocarbons: at a low temperature of 430°C and a working pressure of 7.5 MPa in a reaction mixture, the yield of hydrocarbon products and gases varied from 50.4 to 54.2% with a low yield of high-molecular-weight products (asphaltenes).     

Usefulness of the carbon-13 tracer technique for characterizing terrestrial carbon pools

Application of the carbon-13 (¹³C) tracer technique to soil organic carbon emission is relatively new to many researchers and only a few results have been reported to date. This mini-review paper cites some well-documented research in organic carbon pool studies using the ¹³C tracer technique. The ¹³C abundance in soil usually remains at a stable level under a set of natural conditions. Variations in ¹³C reflect different sources and types of organic components from natural environments. An important feature of ¹³C discrimination in the soil C pool has permitted researchers to assess the dynamic nature of the pool. This discrimination may reflect a selective preference at early stage of residue decomposition by soil microbes. Crop rotation and residue input to humic substances can change the ¹³C abundance, which is a possible way to estimate soil C emission. However, the dynamic relationship between ¹³C abundance in the soil C pool and C emission is still in an early stage of development. Restrictions due to requirement of long-term experiments and duration of vegetation changes may affect its wide-spread adoptions in C emission studies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dipolar-decoupled carbon-13 NMR study of highly oriented polyethylene films

Summary High-resolution solid-state ¹³C NMR spectra and ¹³C relaxation times T₁ and T₁ have been measured at 40–100° C for uniaxially oriented polyethylene films with the drawing direction parallel to the magnetic field; this sample has a unique morphological structure that the noncrystalline chains are nearly disordered irrespective of the high degree of drawing. A sharp resonance line(line A) appears at the position corresponding to the principal value ₃₃ of the chemical shift tensor for the CH₂ carbons with the trans-trans conformation. Another sharp linedine B) is observed at almost the same chemical shift as for the CH₂ carbons of polyethylene in solution. Although these observations are similar to those for a cold-drawn polyethylene sample reported previously, line B is much enhanced in intensity and the linewidth is narrower in the present sample, reflecting the disorientation of the noncrystalline component. It is found from T₁ measurements that line A contains two components with different molecular mobility, both being assigned to the crystalline components. On the other hand, the line B is composed of a single component assignable to the noncrystalline component with liquid-like molecular mobility.     

Determination of the microstructure of acrylonitrile-butadiene and styrene-butadiene copolymers using carbon-13 NMR

Summary The microstructures of copolymers such as acrylonitrile-butadiene and styrene-butadiene are determined directly from the ¹³C NMR spectra without resorting to empirical factors. Experimental parameters were devised to overcome the difficulties due to long spin-lattice relaxation times or to different nuclear Overhauser effects.     

Fully quantitative carbon-13 NMR characterization of resol phenol-formaldehyde prepolymer resins

Different resol phenol-formaldehyde prepolymer resins have been synthesized with different Formaldehyde/Phenol (F/P) ratios or different catalysts and characterized by ¹³C NMR spectroscopy in solution. A fast quantitative measuring protocol is proposed based on the use of chromium(III)acetylacetonate as a relaxation agent. APT (attached proton test) and DEPT (distortionless enhancement by polarisation transfer) spectra were acquired to enable proper resonance assignments, especially in the regions with severe signal overlap. Equations are presented in which the methylene bridges (MB), the methylol groups (MG) and the dimethylene ether bridges (DMEB) of resol resins are quantitatively taken into account. Important structural factors determined quantitatively for resol prepolymer resins are the F/P ratio after reaction, the degree of polymerization (n), the number average molecular weight (M_n) and the content of free ortho and para positions.     

Synthesis gas and char production from Mongolian coals in the continuous devolatilization process

Devolatilization of Mongolian coal (Baganuur coal (BC), Shievee Ovoo coal (SOC), and Shievee Ovoo dried coal (SOC-D)) was investigated by using bench-sized fixed-bed and rotary kiln-type reactors. Devolatilization was assessed by comparing the coal’s type and dry basis, temperature, gaseous flux, tar formation/generation, devolatilization rate, char yield, heating value, and the components of the raw coal and char. In the fixed bed reactor, higher temperatures increased the rate of devolatilization but decreased char production. BC showed higher rates of devolatilization and char yields than SOC or SOC-D. Each coal showed inversely proportional devolatilization and char yields, though the relation was not maintained between the different coal samples because of their different contents of inherent moisture, ash, fixed carbon, and volatile matter. Higher temperatures led to the formation of less tar, though with more diverse components that had higher boiling points. The coal gas produced from all three samples contained more hydrogen and less carbon dioxide at higher temperatures. Cracking by multiple functional groups, steam gasification of char or volatiles, and reforming of light hydrocarbon gas increased with increasing temperature, resulting in more hydrogen. The water gas shift (WGS) reaction decreased with increasing temperature, reducing the concentration of carbon dioxide. BC and SOC, with retained inherent moisture, produced substantially higher amounts of hydrogen at high temperature, indicating that hydrogen production occurred under high-temperature steam. The continuous supply of steam from coal in the rotary kiln reactor allowed further exploration of coal gas production. Coal gas mainly comprising syngas was generated at 700–800 °C under a steam atmosphere, with production greatest at 800 °C. These results suggest that clean char and high value-added syngas can be produced simultaneously through the devolatilization of coal at lower temperature at atmospheric pressure than the entrained-bed type gasification temperature of 1,300–1,600 °C.     

Investigation by carbon-13 and proton NMR spectroscopy of the diglycidyl derivative of phenolphthalein

¹³C- and ¹H-NMR studies have shown conclusively that the main component of the epoxy resin derived from phenolphthalein and epichlorohydrin is an aromatic diglycidyl ether containing a lactone ring, rather than a glycidyl ether-glycidyl ester compound containing a quinoid ring.     

Prospects for carbon-13 nuclear magnetic resonance analysis of solid fossil-fuel materials

Because of excessive line broadening in solids due to magnetic dipole—dipole interactions and chemical shift anisotropies, and because of long spin-lattice relaxation times, standard continuous wave (CW) or pulse Fourier transform techniques do not normally yield structurally informative ¹³C n.m.r. spectra from solid samples. However, the techniques of dipolar decoupling, cross polarization and magic-angle spinning show great promise for routine ¹³C n.m.r. studies of solids. Applications of these techniques to analysis of anthracite, lignite and algal coal samples, and to oil shale and kerogen samples, are discussed. It is shown that cross-polarization spectra obtained with dipolar decoupling display chemical shift anisotropy which interferes with attempts to distinguish the resonances of aromatic and aliphatic carbons. However, with magic-angle spinning the distinction can be made. Prospects and potential difficulties with applications of these techniques are discussed.     

Application of solid state carbon-13 NMR spectroscopy to chemically modified surfaces

The nuclear magnetic resonance (NMR) techniques, high-power decoupling, magic-angle spinning, and cross polarization for high resolution in solid materials have been utilized to characterize surface-modified silicas. Solid state Carbon-13 NMR was used to study the chemical adsorption of a series of organosilane coupling agents on the silica surface. The type of information that can be obtained from such surface studies includes chemical bonding between the surface and the organosilane, the effects on steric hindrance of the hydrocarbon chains of the coupling agent, and other silane-surface interactions. The approach for the analysis involved comparing the spectra of the treated silica to the spectra of the corresponding condensed coupling agents. Spectral differences in terms of changes in chemical shifts and line widths are observed. The results are interpreted in terms of chemical bonding and structural differences of the silane coupling agents adsorbed onto the surface.     

Investigation of ethylene/1-octene copolymerization models by carbon-13 NMR

Statistical reaction models have been used to fit C-NMR spectra for ethylene/1-octene copolymers and to describe the polymerization reaction probabilities. Ten models ranging in complexity from a one-site Bernoulli probability to multiple site second-order Markov systems were studied. Model parameters were determined by fitting the experimental integrations of replicated spectra using a maximum likelihood method. The best fit to the experimental NMR spectra was obtained with a two-site model, one site producing mainly high-density polymer following a Bernoulli probability model, while the second site allows more incorporation of octene following a chain-end controlled probability described by first-order Markov statistics. © 1994 John Wiley & Sons, Inc.     

Chemical characterization of solid coals through magic-angle 13C nuclear magnetic resonance

Recently developed ¹³C n.m.r. techniques for high resolution in solids are applied to native coals. The quantity and distribution of hydroxyl groups in the samples are determined by combining chemical modification through acetylation with the n.m.r. measurements. Spectral subtraction isolates the reaction product resonances. Spectroscopic separation of protonated and non-protonated carbons through pulse programming techniques is also demonstrated. Finally, the susceptibility of the aliphatic carbons in coals to low-temperature oxidation is shown clearly by subtraction of spectra of the oxidized and unoxidized materials.     

Liquefaction of subbituminous coals with a basic nitrogenous model process solvent

Liquefaction reactions in a tubing-bomb reactor have been carried out as a function of coal, coal sampling source, reaction time, atmosphere, temperature, coal pre-treatment, SRC post-treatment and process solvent. Pyridine as well as toluene conversions ranging from 70 to > 90 wt% involving both eastern bituminous and western subbituminous coals are obtained. 1,2,3,4-Tetrahydroquinoline (THQ) has been extensively used as a process solvent under optimized liquefaction conditions of 2:1 solvent: coal, 7.5 MPa H₂, 691 K and 30 min reaction time. Comparisons of THQ with other model process solvents such as methylnaphthalene and tetralin are described. Liquefaction product yield for conversion of subbituminous coal is markedly decreased when surface water is removed from the coal by drying in vacuo at room temperature prior to liquefaction. The effect of mixing THQ with Wilsonville hydrogenated process solvent in the liquefaction of Wyodak and Indiana V coals is described.     

Properties and structure of hard coals from a borehole Niedobczyce IG-1 in the Rybnik Coal District, Upper Silesian Coal Basin, their petrographic and group constituents 3. Changes in the chemical structure of primary tars with increase in coalification degree of the parent coals

The primary tars obtained by the Fischer-Schraeder method from coals described in earlier papers [1] were studied using infrared (i.r.) and ¹H nuclear magnetic resonance (nmr) spectroscopy methods. With the object of achieving a broader evaluation of the properties and structure of these primary tars with increase in coalification of the parent coals—from 78 to 86% C^daf, in the range from gasflame (according to Polish classification) 611-VII (according to international classification and statistical groups) to orthocoking 435-VB coals—they were distilled to the temperature of 543 K under normal pressure. Products obtained were also analysed by the i.r. and ¹H nmr spectroscopy methods, while the distillates/fraction boiling up to 543 K/were additionally analysed by the gas chromatography-mass spectrometry (g.c.-m.s.) and ¹³C nmr methods. A comparative X-ray analysis of the coals and the semi-cokes obtained from them was also conducted.     

Characterization of substituted phenol–formaldehyde resins using solid-state carbon-13 NMR

Crosslinked substituted phenol–formaldehyde resins were synthesized from cashew nut shell liquid, 3-n-pentadecylphenol and phenol with formaldehyde. The resulting resins were crosslinked and investigated using carbon-13 NMR in the solid state using cross-polarization, magic angle spinning, and dipolar decoupling. Comparisons were made between the spectra of pure phenol–formaldehyde resins and it was shown possible to distinguish between the resins. It was also shown that the proton-dephased spectrum gave better spectral resolution for the substituted compounds. In addition, the solids carbon-13 technique verified that the degradation of the substituted phenolic resins occurs first with the degradation of the side chain in agreement with suggestions from earlier work.     

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.