A study of the effect of igneous intrusions on the structure of an Australian high voltage bituminous coal

Samples of heat affected coal taken near two igneous intrusions (dykes) in the Victoria Tunnel Seam of the Newcastle Coalfield have been investigated by chemical analysis, petrography, Fourier transform infrared spectroscopy and solid state¹³C NMR spectroscopy. Visual inspection of the seam near the intrusion showed four distinct zones. These have been called, in order of increasing distance from the dyke, the massive cinder, the banded cinder, heat affected coal and apparently unaffected coal. The samples show an increase in structural change as the distance from the dyke decreases, with the samples taken from the massive cinder nearest the dyke being classified as semicokes.FTIR and¹³C NMR results, together with the results of dipolar dephasing NMR experiments, suggest that the predominant structural change in the heat affected zone is replacement of aromatic ethers and/or phenols with hydrogen, leading to an increase in proton aromaticity. Closer to the intrusion, bond rupture leads to a marked loss of aliphatic groups, while in the most affected region adjacent to the dyke aromatic crosslinking has also occurred to form coke-like material.     

Chemistry of alkali extraction of brown coals—I. Kinetics, characterisation and implications to coalification

Three Australian brown coals have been separated into humin and humic acid fractions and studied by high resolution solid state ¹³C NMR spectroscopy and Fourier transform IR spectroscopy. The aromatic rings of the humic acids are highly substituted showing that extensive cross linking must have occurred during formation from wood lignin and tannin. However, the humins contain more aliphatic carbon and hydrogen than the corresponding humic acids. This shows that little cross linking has occurred with other components of the brown coal such as resins, waxes cutin and algal detritus, and cross linking has not rendered the aromatics alkali insoluble. The kinetics of extraction are complex and not simple first order. This is reflected in the chemical composition of the humic acid which is extraction temperature dependent. We also observed that there is a conversion of aromatic carbon to aliphatic carbon and gas during extraction, probably by alkaline oxidation, resulting in ring opening. A range of suitable model compounds have been studied to confirm this finding. Such a mechanism may account for the modification of lignin in oxidising environments such as those occurring in the initial stages of coalification (lignite or brown coal formation) and in soils.     

Early maturation processes in coal. Part 1: Pyrolysis mass balance and structural evolution of coalified wood from the Morwell Brown Coal seam

We have developed a theoretical approach for evaluating the maturation of kerogen-like material, involving molecular dynamic reactive modelling with a reactive force field to simulate thermal stress. Morwell Brown Coal was selected to study the thermal evolution of terrestrial organic matter (OM). To achieve this, a structural model is first constructed on the basis of literature models and analytical characterization of our samples using modern 1and 2D nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy and elemental analysis. Then, artificial maturation of the coal is performed at low conversion in order to obtain quantitative and qualitative detailed evidence for the structural evolution of the kerogen upon maturation. The chemical changes include defunctionalization of carboxyl, carbonyl and methoxy functional groups, coupled with an increase in cross linking in the residual matured kerogen. Gaseous and liquid hydrocarbons, essentially CH₄, C₄H₈ and C₁₄₊ hydrocarbons, are generated in low amount, merely via cleavage of the lignin side chain.     

Analyses of insoluble residues of altered organic matter by C CP/MAS nuclear magnetic resonance

Alteration phenomena affecting organic matter during diagenesis frequently lead to the formation of residues almost insoluble. Data from ¹³C CP/MAS nuclear magnetic resonance analyses (NMR) of these residues have been compared to those obtained by other techniques such as elemental analysis, infrared spectroscopy, Rock-Eval pyrolysis or gas chromatography. Three examples of alteration phenomena have been chosen: the artificial and natural oxidation of coals, the biodegradation of oils and solid bitumens, and the radiolytic degradation of organic matter. NMR results and those obtained by other techniques converge on similar general conclusions. Additional information can be extracted from ¹³C NMR data: e.g. definition of the phenol/carbonyl ratio, transformation of the aromatic network and aromatic ring substitution. These comparisons are good evidence for the reliability of non-destructive analysis of the insoluble fraction of altered organic material by ¹³C CP/MAS NMR.     

Solid state 13C-NMR spectroscopy of some oligocene coals of Assam and Nagaland

Five representative sub-bituminous coals from NE part of India have been analyzed with respect to their distribution of carbon functional groups through ¹³C solid state NMR experiments. The ¹³C Cross Polarised/Magic Angle Spinning NMR spectra of the coal samples were recorded at temperatures of 23°C, 50°C and 70°C respectively and the changes on their structures were interpreted. The NMR spectra of low rank coals were found to exhibit varying degrees of fine structures. The aromaticities of the coal were also determined from the C/H ratios of coal. Some changes in the peak positions along with the formation of simple aromatic units were observed on mild heating.     

Mineralisation and structural changes during the initial phase of microbial degradation of pyrogenic plant residues in soil

The microbial recalcitrance of char accumulated after vegetation fires was studied using pyrogenic organic material (PyOM) with increasing degrees of charring, produced from rye grass (Lolium perenne) and pine wood (Pinus sylvestris) at 350 °C under oxic conditions. Solid state ¹³C and ¹⁵N nuclear magnetic resonance (NMR) spectroscopy confirmed increasing aromaticity and the formation of heterocyclic N with prolonged charring. After mixing with a mineral soil, the PyOM was aerobically incubated for 48 days at 30 °C. To account for the input of fresh litter after a fire event, unburnt rye grass residue was added as a co-substrate. The grass-derived PyOM showed the greatest extent of C mineralisation. After 48 days incubation, up to 3.2% of the organic C (OC) was converted to CO₂. More severe thermal alteration resulted in a decrease in the total C mineralisation to 2.5% of OC. In the pine-derived PyOM, only 0.7% and 0.5% of the initial C were mineralised. The co-substrate additions did not enhance PyOM mineralisation during initial degradation. ¹³C NMR spectroscopic analysis indicated structural changes during microbial degradation of the PyOM. Concomitant with a decrease in O-alkyl/alkyl-C, carboxyl/carbonyl C content increased, pointing to oxidation. Only the strongly thermally altered pine PyOM showed a reduction in aromaticity. The small C losses during the experiment indicated conversion of aryl C into other C groups. As revealed by the increase in carboxyl/carbonyl C, this conversion must have included the opening and partial oxidation of aromatic ring structures. Our study demonstrates that plant PyOM can be microbially attacked and mineralised at rates comparable to those for soil organic matter (SOM), so its role as a highly refractory SOM constituent may need re-evaluation.     

Molecular level analysis of long term vegetative shifts and relationships to soil organic matter composition

Soil organic matter (SOM) is one of the earth’s largest reservoirs of actively cycled carbon and plays a critical role in various ecosystem functions. In this study, mineral soils with the same parent material and of similar approximate age were sampled from the same climatic region in Halsey, Nebraska to determine the relationship between overlying vegetation inputs to SOM composition using complementary molecular level methods (biomarker analyses and solid state ¹³C nuclear magnetic resonance (NMR) spectroscopy). Soil samples were collected from a native prairie and cedar and pine sites planted on the native prairie. Free and bound lipids isolated from the pine soil were more enriched in aliphatic and cutin-derived compounds than the other two soils. Cinnamyl type lignin-derived phenols were more abundant in the grassland soil than in the pine and cedar soils. Acid to aldehyde ratios (Ad/Al) for vanillyl and syringyl type phenols were higher for the pine soil indicating a more advanced stage of lignin oxidation (also observed by ¹³C NMR) in the soil that has also been reported to have accelerated carbon loss. In agreement with the more abundant aliphatic lipids and cutin-derived compounds, solid state ¹³C NMR results also indicated that the SOM of the pine soil may have received more aliphatic carbon inputs or may have lost other components during enhanced decomposition. The observed relationship between vegetation and SOM composition may have important implications for global carbon cycling as some structures (e.g. aliphatics) are hypothesized to be more recalcitrant compared to others and their accumulation in soils may enhance below ground carbon storage.     

Comparative characterization of humic substances from the open ocean, estuarine water and fresh water

Humic substances were isolated from ocean, estuarine water and fresh water using a two column array of XAD-8 and XAD-4 resins in series. The extracted fulvic acids and XAD-4 fraction from different origins were characterized using UV–vis., molecular fluorescence, Fourier transform infrared (FTIR) spectroscopy and cross polarization magic angle spinning (CPMAS)-¹³C nuclear magnetic resonance (NMR) spectroscopy. The isolation procedure allowed us to obtain the necessary amount of sample for characterization, even in the case of open ocean water, which has a very low amount of dissolved organic carbon (DOC). Humic substances from the open ocean showed the lowest chromophore and fluorophore contents and showed relatively greater fluorescence at lower wavelengths than those from fresh water. FTIR and ¹³C NMR spectra highlighted the idea that humic substances from a marine environment have a more branched aliphatic structure and less aromatic structure than those highly influenced by terrestrial sources. The spectra also suggest that the open ocean humic substances have a higher content of olefinic carbons than aromatic- or alkyl-substituted carbons.     

Chemical structures of coal lithotypes before and after CO2 adsorption as investigated by advanced solid-state C nuclear magnetic resonance spectroscopy

Four lithotypes (vitrain, bright clarain, clarain, and fusain) of a high volatile bituminous Springfield Coal from the Illinois Basin were characterized using advanced solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. The NMR techniques included quantitative direct polarization/magic angle spinning (DP/MAS), cross polarization/total sideband suppression (CP/TOSS), dipolar dephasing, CH_n selection, and recoupled C-H long-range dipolar dephasing techniques. The lithotypes that experienced high-pressure CO₂ adsorption isotherm analysis were also analyzed to determine possible changes in coal structure as a result of CO₂ saturation at high pressure and subsequent evacuation. The main carbon functionalities present in original vitrain, bright clarain, clarain and fusain were aromatic carbons (65.9%-86.1%), nonpolar alkyl groups (9.0%-28.9%), and aromatic C-O carbons (4.1%-9.5%). Among these lithotypes, aromaticity increased in the order of clarain, bright clarain, vitrain, and fusain, whereas the fraction of alkyl carbons decreased in the same order. Fusain was distinct from other three lithotypes in respect to its highest aromatic composition (86.1%) and remarkably small fraction of alkyl carbons (11.0%). The aromatic cluster size in fusain was larger than that in bright clarain. The lithotypes studied responded differently to high pressure CO₂ saturation. After exposure to high pressure CO₂, vitrain and fusain showed a decrease in aromaticity but an increase in the fraction of alkyl carbons, whereas bright clarain and clarain displayed an increase in aromaticity but a decrease in the fraction of alkyl carbons. Aromatic fused-rings were larger for bright clarain but smaller for fusain in the post-CO₂ adsorption samples compared to the original lithotypes. These observations suggested chemical CO₂-coal interactions at high pressure and the selectivity of lithotypes in response to CO₂ adsorption.     

煤红外光谱的精细解析及其煤化作用机制

应用傅立叶红外光谱(FTIR)分析技术,采用计算机曲线拟合方法对红外光谱进行分峰处理,通过对自然演化系列和人工模拟实验系列样品的研究,定量分析了不同煤化程度煤的红外光谱特征。研究表明,煤的红外光谱在一定程度上可以定量表征煤化作用程度。含氧基团和烷基侧链随煤化程度的增高以不同的速度发生脱落,且脱落的速度具有明显的阶段性:羧基的减少主要发生在R_{max为0.5%之前,但可以持续到肥煤阶段;脂肪基团的脱落主要发生在沥青化作用阶段内,并导致生成大量的烃类,沥青化作用之前发生的脂肪基团的脱落是煤的未熟和低熟油形成的主要原因;在沥青化阶段之后的高演化阶段,煤中的烷基侧链已经脱落殆尽,芳环缩合程度明显提高。红外光谱的分峰技术和定量,为精细解析煤的结构特征和煤化作用研究提供了一种新的方法。}      

The complementary use of H NMR, C NMR, FTIR and size exclusion chromatography to investigate the principal structural changes associated with composting of organic materials with diverse origin

The aim of this work is to study the structural changes involved in humification processes. Total humic extracts (THE) obtained from five composted materials of diverse origin (solid wastes of wineries, solid mill olive wastes, domestic wastes, ovine manures plus straw, and a mixture of animal manures), and their corresponding initial raw fresh organic mixtures were studied using ¹³C nuclear magnetic resonance (NMR) using the cross-polarization magic angle spinning technique (CPMAS), ¹H NMR, Fourier transform infrared spectroscopy (FTIR) and high pressure size exclusion chromatography (HPSEC). One group of three humic acids extracted from soils, and a second group consisting of two reference humic acids and two reference fulvic acids (1S104H, 1R103H, 1R101F and 1R107F) obtained from the International Humic Substances Society were also characterized using these techniques, in order to compare the features of reference humic and fulvic acids with those of composted materials. Likewise, the results were compared with those obtained in previous studies, in which UV–Visible and fluorescence spectroscopies were employed to characterize the humification degree of the molecular systems.The results obtained by ¹³C CPMAS NMR, ¹H NMR and FTIR indicate that, in general, humification seems to be associated with an increase in the aromatic character of the systems, with the presence of phenol groups as principal substituents and a reduction in oxygen containing functional groups, principally carboxylic or carbonylic groups, as well as the development of molecular fractions with larger size. These results also support the suitability of UV–Visible and fluorescence spectroscopies in the assessment of the humification course of humic extracts in composting processes.     

Compositional diversity in insoluble organic matter in type 1, 2 and 3 chondrites as detected by infrared spectroscopy

Insoluble organic matter (IOM) isolated from 22 carbonaceous and ordinary chondrites spanning a wide range of groups and petrologic types were analyzed using Fourier transform infrared spectroscopy (FTIR). Based on common IR spectral features, it is observed that IOM falls into 4 molecularly distinct groups (designated here as A through D). Spectral group A includes type 1 and 2 chondrites and exhibits intense aliphatic C-H and carboxyl vibrational peaks. Spectral group B includes the least metamorphosed type 3 chondrites and Tagish Lake, and exhibits weaker aliphatic and carboxyl vibrational intensity. Spectral groups C and D include metamorphosed type ?3.1 chondrites and a heated CM chondrite. The carbonyl stretching features in spectral groups C and D differ from that in spectral groups A and B and from each other. In spectral group C, the carbonyl stretching is assigned to cyclic unsaturated lactones; in spectral group D carbonyl exists predominantly in the form of unsaturated ketone moieties. Both spectral groups C and D have a relatively narrow band structure around 1210 cm⁻¹ (assigned to aromatic skeletal modes) as compared with spectral groups A and B, which is consistent with the formation of more condensed aromatics by extensive thermal metamorphism. The differences in carbonyl structures in spectral groups C and D are not the result of different effective metamorphic temperatures, rather these differences likely result from variation in the activity of water and oxygen at different stages of parent body metamorphism. Such environmental variations must be local phenomena in the parent bodies as there is no correlation between spectral grouping and chondrite class or group.     

Exploitation of relaxation in cross-polarization nuclear magnetic resonance spectroscopy of fossil fuels

The judicious choice of dipolar dephasing times or carbon magnetization holding times has been shown to improve resolution in solid state nuclear magnetic resonance (NMR) spectra of complex materials. Signals from protonated and alkylated aromatic carbons are reduced to enhance resolution of aromatic oxygenated groups. Rapidly rotating methyl groups can be resolved from other aliphatic carbon types. These techniques were used to investigate the structure of a brown coal, xylite fractions of a brown coal, a bituminous coal, an oil shale and a solvent-refined coal. The results allow estimates of the fraction of aromatic carbon that is protonated in coal to be made, and demonstrate that methyl groups in coal rotate rapidly at room temperature.     

Methylation patterns of aquatic humic substances determined by13C NMR spectroscopy

¹³C NMR spectroscopy is used to examine the hydroxyl group functionality of a series of humic and fulvic acids from different aquatic environments. Samples first are methylated with¹³C-labeled diazomethane. The NMR spectra of the diazomethylated samples allow one to distinguish between methyl esters of car☐ylic acids, methyl ethers of phenolic hydroxyls, and methyl ethers of phenolic hydroxyls adjacent to two substituents. Samples are then permethylated with¹³C-labeled methyl iodide/NaH.¹³C NMR spectra of permethylated samples show that a significant fraction of the hydroxyl groups is not methylated with diazomethane alone. In these spectra methyl ethers of carbohydrate and aliphatic hydroxyls overlap with methyl ethers of phenolic hydroxyls. Side reactions of the methyltion procedure including carbon methylation in the CH₃I/NaH procedure, are also examined. Humic and fulvic acids from bog, swamp, groundwater, and lake waters showssome differences in their distribution of hydroxyl groups, mainly in the concentrations of phenolic hydroxyls, which may be attributed to their different biogeochemical origins.     

固体13C核磁共振技术在石油地球化学研究中的应用

生烃母质的结构及其演化特征是石油地球化学研究的重要课题之一,随着新的分析测试方法的出现,生烃母质结构组成的研究已从基本的元素分析向官能团分析方向发展。固体核磁共振¹³C（NMR）分析已成为研究物质结构及其演化的一种有效手段,它可以测定不同类型含碳官能团的定量分布,从而计算各种结构参数,提供有关碳骨架最直接的重要信息。将这些结构信息从烃源岩的生烃潜力评价、烃源岩的生烃演化过程及机理、烃源岩的热演化判定等角度考虑实际存在的地质问题,发现脂族碳是油气的主要贡献者,油气潜力碳含量的高低决定了烃源岩生烃潜力的大小,芳核平均结构尺寸是有效的热演化衡量标尺。固体¹³C核磁共振分析为石油地球化学的研究打开了新局面。     

Primary alteration—oxidation of marine algal organic matter from oil source rocks of the North Sea and Norwegian Arctic: new findings

The presence of partially oxidized algal organic matter in oil-prone marine source rocks, is the rule rather than the exception. Partially oxidized, algal kerogen can still act as a significant source of liquid hydrocarbons. However, the corresponding peak of C₁₂ + hydrocarbon generation is shifted to a considerably lower maturity level compared with that of the classical Type II kerogen. The extent of primary alteration-oxidation of marine algal kerogen is monitored by means of solid state microfluorescence spectroscopy. A new parameter, the Primary Alteration Factor (PAF) is established, and the relationships between PAF and H/C, O/C, HI, TOC and between PAF and %₀δ¹³C are determined. The present data show large variations in the bulk chemistry of immature marine algal kerogens, and reveal evidence for gradational dehydrogenation/oxidation of the source organic matter. This contrasts with the recently proposed mechanism for kerogen formation. SEM analysis reveals a relationship between the physical breakdown of algal organic matter and the formation of liptodetrinite. FTIR analysis shows that the incorporation of primary oxygen in the kerogen macromolecules is not in the form of carbonyl or carboxyl functionalities. The presence of highly unreactive, stable oxygen, associated with aromatic structures in partially oxidized algal kerogen, is suggested by resistance of the kerogen to graphitization. The FTIR data also suggest the presence of aryl ether oxygen. The present findings raise fundamental questions regarding the mechanisms of kerogen cracking and kerogen formation, and have important implications for petroleum exploration.     

Methodological comparison for quantitative analysis of fossil and recently derived carbon in mine soils with high content of aliphatic kerogen

In mine soil, quantification of soil organic carbon (OC) derived recently from biomass decomposition is complicated by the presence of fossil (geogenic) C derived from coal, oil shale, or similar material in the overburden. The only reliable method for such measurement is ¹⁴C analysis (i.e. radiocarbon dating) using instrumentation such as accelerator mass spectrometry, which is too expensive for routine laboratory analysis. We tested two previously used and two new methods for recent C quantification and compared them with ¹⁴C AMS radiocarbon dating as a reference using a set of soil samples (n = 14) from Sokolov, Czech Republic: (i) ¹³C isotope ratio composition, (ii) cross polarization magic angle spinning ¹³C nuclear magnetic resonance (CPMAS ¹³C NMR) spectroscopy, (iii) near infrared spectroscopy (NIRS) coupled with partial least squares regression and (iv) Rock–Eval pyrolysis. Conventional methods for OC determination (dry combustion, wet dichromate oxidation, loss-on-ignition) were also compared to quantify any bias connected with their use. All the methods provided acceptable recent carbon estimates in the presence of mostly aliphatic fossil C from kerogen. However, the most accurate predictions were obtained with two approaches using Rock–Eval pyrolysis parameters as predictors, namely (i) S₂ curve components and (ii) oxygen index (OI). The S₂ curve approach is based on the lower thermal stability of recent vs. fossil organic matter. The OI approach corresponded well with ¹³C NMR spectra, which showed that samples rich in recent C were richer in carboxyl C and O-alkyl C. These two methods showed the greatest potential as routine methods for recent C quantification.     

NMR studies of chemical structural variation of insoluble organic matter from different carbonaceous chondrite groups

Solid-state ¹H and ¹³C Nuclear Magnetic Resonance (NMR) spectroscopic experiments have been performed on isolated meteoritic Insoluble Organic Matter (IOM) spanning four different carbonaceous chondrite meteorite groups; a CR2 (EET92042), a CI1 (Orgueil), a CM2 (Murchison), and the unique C2 meteorite, Tagish Lake. These solid state NMR experiments reveal considerable variation in bulk organic composition across the different meteorite group’s IOM. The fraction of aromatic carbon increases as CR2 < CI1 < CM2 < Tagish Lake. The increases in aromatic carbon are offset by reductions in aliphatic (sp³) carbon moieties, e.g., “CH_x,” and “CH_x(O,N).” Oxidized sp² bonded carbon, e.g., carboxyls and ketones grouped as “CO,” are largely conservative across these meteorite groups. Single pulse (SP) ¹³C magic angle spinning (MAS) NMR experiments reveal the presence of nanodiamonds with an apparent concentration ranking in the IOM of CR2 < CI1 < CM2 < Tagish Lake. A pair of independent NMR experiments reveals that, on average, the aromatic moieties in the IOM of all four meteoritic IOM fractions are highly substituted. Fast spinning SP ¹H MAS NMR spectral data combined with other NMR experimental data reveal that the average hydrogen content of sp³ bonded carbon functional groups is low, requiring a high degree of aliphatic chain branching in each IOM fraction. The variation in chemistry across the meteorite groups is consistent with alteration by low temperature chemical oxidation. It is concluded that such chemistry principally affected the aliphatic moieties whereas the aromatic moieties and nanodiamonds may have been largely unaffected.     

桂中晚二叠世碳酸盐岩碳同位素的地层学意义

广西合山马滩等地的上二叠统剖面碳酸盐岩碳同位素在吴家坪阶和长兴阶界线处表现出明显的降低现象,吴家坪阶石灰岩样品δ１３Ｃ值一般都高于３．２‰,而长兴阶的样品其δ１３Ｃ值多低于３．２‰,因此,碳同位素组成的差异有可能作为吴家坪阶和长兴阶的同位素地层界线。这种同位素降低现象可能与澳洲、南非、印度等地大规模聚煤作用的中止有关。聚煤作用的中止以及以前埋藏的泥炭和煤层在遭受构造抬升和氧化后会向大气中释放出大量富含轻同位素１２Ｃ的ＣＯ_２,从而使得与大气平衡的海水及在其中沉淀出的碳酸盐岩的δ１３Ｃ值降低。     

Effects of Coal Rank and High Organic Sulfur on the Structure and Optical Properties of Coal-based Graphene Quantum Dots

Coal‐based graphene quantum dots (GQDs) were successfully produced via a one‐step chemical synthesis from six different coal ranks, from which two superhigh organic sulfur (SHOS) coals were selected as natural S‐doped carbon sources for the preparation of S‐doped GQDs. The effects of coal properties on coal‐based GQDs were analyzed by means of high‐resolution transmission electron microscopy (HRTEM), X‐ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X‐ray photoelectron spectroscopy (XPS), ultraviolet‐visible (UV‐Vis) absorption spectroscopy, and fluorescence emission spectra. It was shown that all coal samples can be used to prepare GQDs, which emit blue‐green and blue fluorescence under ultraviolet light. Anthracite‐based GQDs have a hexagonal crystal structure without defects, the largest size, and densely arranged carbon rings in their lamellae; the high‐rank bituminous coal‐based GQDs are relatively reduced in size, with their hexagonal crystal structure being only faintly visible; the low‐rank bituminous coal‐based GQDs are the smallest, with sparse lattice fringes and visible internal defects. As the metamorphism of raw coals increases, the yield decreases and the fluorescence quantum yield (QY) initially increases and then decreases. Additionally, the surface of GQDs that were prepared using high‐rank SHOS coal (high‐rank bituminous coal) preserves rich sulfur content even after strong oxidation, which effectively adjusts the bandgap and improves the fluorescence QY. Thus, high‐rank bituminous coal with SHOS content can be used as a natural S‐doped carbon source to prepare S‐doped GQDs, extending the clean utilization of low‐grade coal.