Nonprotonated Aromatic Carbons Research Articles

Role of aromatic and non-protonated aromatic carbon in the stability of soil organic matter

Research on soil organic matter (SOM) in its natural state is vital for comprehending the mechanisms governing soil stability and carbon cycling, crucial in addressing global climate change. We utilized solid-state 13C nuclear magnetic resonance (NMR) spectroscopy on HF treated samples, along with a 260-day laboratory mineralization experiment and thermal analysis-programmed pyrolysis (PP) to evaluate SOM biodegradability and thermal stability. To cover the potential range of organic carbon variability, we selected samples from three land uses and soil types, featuring total organic carbon levels from 1% to 39%. Our analysis confirmed the substantial contribution of non-protonated aromatic-carbon not bonded to oxygen to SOM’s biological and thermal stability, constituting approximately 14–21% of soil organic carbon. These components exhibited a strong correlation with SOM stability matrices, such as thermal stability and oxygen index determined by PP. Samples with a higher prevalence of these components also displayed the lowest cumulative carbon mineralization. These findings enhance our understanding of the stable SOM pool, aiding in the identification of sustainable soil management practices to mitigate climate change impacts on soil health and carbon dynamics.

Pyrolysis characteristics and kinetic analysis of coating pitch derived from ethylene tar using model-free and model-fitting methods

This work conducted the pyrolysis of coating pitch derived from the ethylene tar through thermogravimetry analysis. The thermogravimetry-mass spectrometry and solid-state 13C nuclear magnetic resonance spectroscopy were performed to correlate the structural characteristics with the pyrolysis behavior. Meanwhile, the pyrolysis kinetics were estimated with model-free and model-fitting methods. The TG-DTG results showed that the pyrolysis process was generally divided into drying, fast pyrolysis, and polycondensation stages, accompanied by the decomposition of active functional groups like the aliphatic carbons bonded to oxygen (falO), the pyrolysis of relatively stable groups like the non-protonated aromatic carbon (faH), and the condensation of high bonding-energy aromatic structure like aromatic bridgehead carbon (faB). The activation energy obtained from model-free analysis ranged from 78.12 to 150.92 kJ/mol with the increasing conversion, indicating the pyrolysis process as a multi-step reaction mechanism. Furthermore, the reaction model A1/3 (gα=[−ln(1−α)]3) was identified as the most suitable model for the pyrolysis of coating pitch in the conversion range of 0.25–0.8. The modeling values matched well with the experimental data, indicating the accuracy and feasibility of the results.

Chemical composition of soil humin in an organic soil profile

Humin is identified as the most recalcitrant fraction of soil organic matter, but its synthesis and depth variability in the soil profile are unknown. Here, we suggest that carboxyl-rich alicyclic structures (CRAM) and fused aromatic ring structures (FAR) can process significant components of the humin structure. We conducted molecular-level characterization of humin using advanced solid-state nuclear magnetic resonance spectroscopy (SSNMR) experiments and calculated the proportion of FAR and functionalities of CRAM (OCq, –C(CH)n, –CH3) structures. Dipolar dephased (dd) direct polarization magic angle spinning (DPMAS) NMR experiments provided insights into non-protonated carbon and mobile quaternary carbons. We calculated the proportions of non-protonated aromatic carbon and non-polar aliphatic C in the organic layers of the topsoil (Oa), mid-soil (Om), and mineral subsoil (Cg). It was evident that the total aliphatic carbon constitutes a significant proportion of humin, as is the non-protonated C attached to CH3. The 13C dd/DPMAS NMR spectrum of Cg humin clearly showed the formation and preservation of hydrophobic OCq, which is indicative of CRAM structures. CRAM structures could be attached to FAR, which makes the whole structure relatively more hydrophobic, a critical humin functionality in deeper soil. The findings from our research can be successfully utilized in terrestrial carbon sequestration studies to amplify the potential of humin as a recalcitrant carbon reservoir.

Study on the Wetting Mechanisms of Different Coal Ranks Based on Molecular Dynamics

The exploration of coal wettability is not only of paramount significance in the mitigation of coal dust and the development of coalbed methane, but it also provides crucial technical support for realizing the geological storage of CO2 within the ‘dual-carbon’ background. Molecular simulation serves as an effective means by which to investigate coal wettability at the microscopic level. This study employed a molecular dynamics simulation to investigate the wettability of coal across 13 distinct coal ranks. Through the analysis of trajectory files, and the incorporation of experimental data during the modeling process, the mechanisms governing the evolution of wettability were revealed. The results demonstrated that the contact angle on the surface of coal increases with the elevation of coal rank. The molecule relative concentration analysis revealed that, with increasing coal rank, the overlap range between water droplets and the coal slab decreases, the height increases, and the diffusion degree of water molecules decreases, which are outcomes consistent with the results of the contact angle measurement. The contact angle was strongly correlated with the number of hydrogen bonds and secondarily correlated with the numbers of carbonyls, hydroxyls, and ether oxygens. The formation of hydrogen bonds was notably correlated with the number of hydroxyls, followed by that of ether oxygens, while its correlations with carbonyls and carboxyls were comparatively weaker. The contact angle exhibited positive correlations with vitrinite reflectance and carbon content, while showing negative correlations with oxygen content, H/C, and O/C. Additionally, it demonstrated positive associations with total sp2 carbon (fa), aromatic carbon (fa′), and non-protonated aromatic carbon (faN), and negative associations with aliphatic carbon (fal) and methylene carbon (falH). Understanding the variations in wettability among different coal ranks can provide a foundational model and theoretical basis for further exploration of the complex interactions among coal, gas, and water across various coal ranks.

Solid-state 13C NMR spectroscopic study of supercritical CO2 catalyzation treated polyethylene terephthalate textiles for platinum metallization

In this study, polyethylene terephthalate (PET) textiles were metallized with Pt via supercritical carbon dioxide (sc-CO2) catalyzation, using palladium(II) hexafluoroacetylacetonate (Pd(hfac)2) as the Pd catalyst source. The effects of the treatments on the structural changes and molecular mobility of PET were elucidated by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. The carbonyl and nonprotonated aromatic carbons and the methylene and protonated aromatic carbons were analyzed from the NMR spectra using a three-component model (crystalline, rigid amorphous, and mobile amorphous components), which inferred that the PET polymer textile comprised one crystalline and two amorphous regions. Additionally, when Pd catalyzation was performed on the textile under sc-CO2 conditions, the crystalline peak of the carbonyl carbon exhibited a chemical shift into a higher magnetic field, and the spin–lattice relaxation time increased. These results implied that Pd could exist in the vicinity of the CO carbon. The X-ray diffraction studies also showed crystallinity of PET increased by sc-CO2 catalyzation, while sc-CO2 annealing without Pd(hfac)2 did not increase it. Therefore, Pt films were successfully coated on the PET via catalytic plating using sc-CO2. The catalyzation reduced the molecular mobility of the PET polymer, indicating the incorporation of the catalytic metal into the polymer chain. The carbonyl carbon peak simultaneously shifted to a higher field, implying that the catalytic Pd could approach the carbonyl groups. The detailed analysis of the structure and molecular mobility provides valuable information that can contribute to the development of novel conductive materials.

Effect of different functional groups on CH4 adsorption heat and surface free energy of vitrinite during coalification

Here vitrinite of various coal ranks was collected to study the characteristics of initial isosteric heat of adsorption (Qst0) and surface free energy (Δσ) by the isothermal methane-adsorbing at various temperatures on dry basis. The surface area of demineralized dry samples was examined for the calculation of Qst0 and Δσ, and the macromolecular structures of dry vitrinite samples were determined using Fourier transform infrared spectroscopy and 13C nuclear magnetic resonance experiments. The Qst0 shows no obvious correlations with oxygen-containing groups, however, there is a positive correlation between the alcoholic hydroxyl group (R–OH) and maximal surface free energy (Δσmax), indicating that the decreasing in R–OH content with the increasing coal rank can reduce adsorption capacity. Qst0 decrease as the amount of nonprotonated aromatic carbon attached to phenolic (including phenoxy or phenol groups) increases, indicating that phenoxy or phenol group loss with maturation is beneficial to increasing Qst0. As the degree of coal metamorphism increases, the aliphatic side chains gradually decline, inducing Qst0 initially decreases and then increases. For aromatic groups, with increases of aromatic carbons, aromatic bridgehead carbons, and ratio of aromatic CC to aromatic CH, Qst0 initially decreases and then increases.

Study on the structure characteristics and gasification activity of residual carbon in biomass ashes obtained from different gasification technologies

The structure characteristics and gasification activity of residual carbons in two rice husk (RH) gasification ashes, a fly ash (FAC) obtained from industrial circulating fluidized bed gasifier (CFBG) and a bottom ash (BAU) obtained from industrial updraft fixed bed gasifier (UFBG), were investigated. Results show that the residual carbon content of FAC (15.93%, dry basis) is much lower than that of BAU (40.27%, dry basis), and the carbon conversion efficiencies of RH in CFBG and UFBG are 93.09% and 74.44%, respectively. The dominant carbon species in RH is oxygen-linked aliphatic carbon (ca. 80%), which disappears in the residual carbons of FAC and BAU. Consequently, the nonprotonated aromatic carbon in RH increases significantly from about 3% to 66–68% during gasification processes, leading to the aromaticity of carbon increases from about 15% to 88–90%. The surface area and pore volume of carbon in RH increase significantly during gasification processes, especially those of FAC, while the gasification activity of carbon in RH decreases slightly due to the increased aromaticity of carbon.

Effects of the Chemical Structure, Surface, and Micropore Properties of Activated and Oxidized Black Carbon on the Sorption and Desorption of Phenanthrene.

The effects of the chemical structure, surface properties, and micropore of modified black carbon samples (BCs) on the sorption mechanism of hydrophobic organic contaminants (HOCs) are discussed. Activated and oxidized BCs were produced from a shale kerogen at 250-500 °C by chemical activation regents (KOH and ZnCl2) and then by oxidative regents (H2O2 and NaClO). The surface properties (water contact angel, Boehm titration, and cation exchange capacity, CEC), structural properties (advanced solid-state 13C NMR), micropore properties (CO2 adsorption), mesopore properties (N2 adsorption), and sorption and desorption properties of phenanthrene were obtained. The results showed that ZnCl2-activated BCs had higher basic surface groups, CEC values, aromatic carbon contents, micropore volumes, and adsorption volumes but exhibited lower acidic surface groups than the KOH-activated BCs did. Micropore modeling and sorption irreversibility indicated that the micropore filling was the main sorption mechanism of phenanthrene. In addition, ZnCl2 activated and NaClO oxidized BCs showed a nice regression equation between adsorption volumes and micropore volumes (CO2- V0) as follows: Q0' = 0.495 V0 + 6.28( R2 = 0.98, p < 0.001). Moreover, the contents of nonprotonated aromatic carbon, micropore volumes, and micropore sizes are the critical factors to micropore filling mechanism of phenanthrene on BCs. The size of fused aromatic rings was estimated from the recoupled 1H-13C dipolar dephasing, and the BC structural models at temperatures ranging from 300 to 500 were proposed. This finding improves our understanding of the sorption mechanism of HOCs from the perspectives of chemical structure and micropore properties.

Investigation into the Effect of Heteroatom Content on Kerogen Structure Using Advanced 13C Solid-State Nuclear Magnetic Resonance Spectroscopy

To elucidate how different extreme heteroatom concentrations in oil shale kerogen may present and contribute to various structural features, three shale samples, containing kerogen with high oxygen content, low heteroatom content, and high sulfur content, were analyzed using advanced 13C solid-state nuclear magnetic resonance (NMR) techniques, including multiple cross-polarization/magic angle spinning (multiCP/MAS), dipolar dephasing (multiCP/DD), and 2D 1H–13C heteronuclear correlation (2D HETCOR). We found that oxygen in Estonian kukersite was present mostly in aromatic C–O structures and that nonprotonated aromatic carbons bonded to oxygen and alkyl chains led to more diverse aromatic signal distributions and structures in the kukersite organic matter than were observed in the other shales. The low-heteroatom kerogen present in Australian Glen Davis torbanite had the simplest structural pattern and the lowest aromaticity, despite having a lower atomic H/C ratio than the kerogens present in the other sh...

The systematic characterization of nanoscale bamboo charcoal and its sorption on phenanthrene:A comparison with microscale

This study investigated the characteristics of nanoscale bamboo charcoal (NBC), and made a comparison with microscale bamboo charcoal (MBC) on how they impact on the sorption abilities of different soils. The two charcoals contained similar elemental contents (e.g., high C, low H and low N) and various functional groups on their surfaces (e.g., aromatic structure, carboxyl, and hydroxyl). However, NBC had a larger total pore volume than that of MBC and was more likely to generate multi-layer sorption of phenanthrene. Controlled by van der Waals forces and electrostatic forces, NBC formed meso-and macropores (intra-particle porosity) and a more intricate pore structure. The performance of NBC in aqueous and soil-water systems was conspicuous and impressing. In aqueous system, by virtue of its larger pore volume, surface area and nonprotonated aromatic carbon, the Kd (sorption coefficient) of NBC reached up to 1.24×106, almost 10 times higher than that of MBC. In soil-water systems, although it could aggregate and react with compounds in soil, the performance of NBC was not weakened by the complicated soil properties, and was still more capable of phenanthrene sorption than MBC, even at an extremely low addition rate 0.2% in soils. Additionally, in comparison with some other common biochars, NBC still showed a promising capacity for phenanthrene sorption in two systems. This finding increases our knowledge of NBC for the remediation of organic pollutants in soil and indicates that the addition rate of charcoals in soils could be reduced by lessening the particle size. Therefore, NBC provides a new possibility for soil pollutant remediation and deserves further research.

Composition of clay-fraction organic matter in Holocene paleosols revealed by advanced solid-state NMR spectroscopy

Identifying which components of organic matter are preserved over millennia in paleosols is important for understanding the stabilization mechanisms of soil organic matter (SOM) and in evaluating the portion of the terrestrial carbon stock that might be attributed to paleosols. Using advanced solid-state nuclear magnetic resonance (NMR) techniques, we examined the chemical composition of clay-associated SOM in a chronosequence of four paleosols and one modern soil collected from a site in the eastern Great Plains in Kansas, USA. The NMR spectra of clay SOM of the paleosols indicated the presence of OCH and CH2 moieties, which represent polysaccharides and lipids, respectively. The paleosol SOM also showed an appreciable peptide component, as indicated by signals of NCH and NCO. As in the modern soil, lignin residues in the paleosols contributed little to the SOM, and condensed aromatics contributed modestly to the clay SOM. The Roberts Creek paleosol samples were exceptions. Their NMR spectra included a considerable nonprotonated aromatic carbon component, reflecting fused aromatic rings and suggesting the presence of clay-fraction charcoal residues, probably originating from paleofires. In the clay SOM of both the paleosols and the modern soil, the polysaccharides and peptides indicated by the NMR spectra are consistent with the presence of microbially derived peptidoglycan and perhaps chitin residues. We conclude that lipids, charcoal, and polysaccharides, probably in the form of microbial residues, are likely to be the dominant SOM components in the paleosol clay fractions.

Effects of Biomass Types and Carbonization Conditions on the Chemical Characteristics of Hydrochars

Effects of biomass types (bark mulch versus sugar beet pulp) and carbonization processing conditions (temperature, residence time, and phase of reaction medium) on the chemical characteristics of hydrochars were examined by elemental analysis, solid-state ¹³C NMR, and chemical and biochemical oxygen demand measurements. Bark hydrochars were more aromatic than sugar beet hydrochars produced under the same processing conditions. The presence of lignin in bark led to a much lower biochemical oxygen demand (BOD) of bark than sugar beet and increasing trends of BOD after carbonization. Compared with those prepared at 200 °C, 250 °C hydrochars were more aromatic and depleted of carbohydrates. Longer residence time (20 versus 3 h) at 250 °C resulted in the enrichment of nonprotonated aromatic carbons. Both bark and sugar beet pulp underwent deeper carbonization during water hydrothermal carbonization than during steam hydrothermal carbonization (200 °C, 3 h) in terms of more abundant aromatic C but less carbohydrate C in water hydrochars.

Structure and composition of hard coke deposited on industrial fluid catalytic cracking catalysts by solid state 13C nuclear magnetic resonance

Carbonaceous deposits (hard coke) were studied by various solid state 13C nuclear magnetic resonance (NMR) techniques after demineralization of the spent and regenerated fluid catalytic cracking (FCC) catalysts obtained from Indian refineries. A number of structural parameters such as aromaticity, H/C ratio, fraction of protonated (faP) and non-protonated (faNP) aromatic carbons, number of pericondensed rings per average molecule (Nperi), and aromatic condensation index (γar) are derived from the NMR data. Findings of thirty-two and thirty-nine aromatic rings in coke from regenerated catalysts and five and nineteen rings in coke from spent catalysts are rationalized by various parameters such as feed, temperature and process conditions of FCC reactor on the basis of condensation. The coke are found more condensed in regenerated catalysts compared to spent catalysts revealing that the temperature has a marked effect in the evolution of coke structure whereas feeds and other process conditions govern the nature and composition of coke.

Determination of the Average Aromatic Cluster Size of Fossil Fuels by Solid-State NMR at High Magnetic Field

We show that the average aromatic cluster size in complex carbonaceous materials can be accurately determined using fast magic-angle spinning (MAS) NMR at a high magnetic field. To accurately quantify the nonprotonated aromatic carbon, we edited the 13C spectra using the recently reported MAS-synchronized spin–echo, which alleviated the problem of rotational recoupling of 1H-13C dipolar interactions associated with traditional dipolar dephasing experiments. The dependability of this approach was demonstrated on selected Argonne Premium coal standards, for which full sets of basic structural parameters were determined with high accuracy.

Transformation of lignocellulosic biomass during torrefaction

In this study, the effect of torrefaction on the chemical and structural transformation of lignocellulosic biomass was investigated using complementary analytical tools. It was observed that the acid-insoluble fraction was increased from approximately 30 to 38% and the methoxyl content was decreased to about half after torrefaction at 330°C for 2.5min. These results highlight the formation of condensed structures along with lignin transformation via demethoxylation. Solid-state NMR spectroscopy indicated that upon torrefaction the aromaticity increased from about 36 to 60%. For the sample torrefied at 330°C, the non-protonated aromatic carbon fraction was found to be about 60% of total aromatic carbons, indicating the formation of large aromatic clusters. The complementary analyses used in this study are proposed as a suitable approach for the elucidation of chemical and structural transformation of biomass during thermal treatment.

Spectral editing in 13C solid-state NMR at high magnetic field using fast MAS and spin-echo dephasing

A simple method is proposed for separating NMR resonances from protonated and non-protonated aromatic carbons in solids under fast magic angle spinning (MAS). The approach uses a MAS-synchronized spin-echo to exploit the differences in rotational recoupling of the dipolar interactions while fully refocusing the isotropic chemical shifts. This strategy extends the relevant time scale of spin evolution to milliseconds and circumvents the limitation of the traditional dipolar dephasing method, which in fast rotating solids is disrupted by rotational refocusing. The proposed approach can be used for quantitative measurement of carbon aromaticities in complex solids with poorly resolved spectra, as demonstrated for model compounds.

Kinetics and reaction chemistry for slow pyrolysis of enzymatic hydrolysislignin and organosolv extracted lignin derived from maplewood

The kinetics and reaction chemistry for the pyrolysis of Maplewood lignin were investigated using both a pyroprobe reactor and a thermogravimetric analyser mass spectrometry (TGA-MS). Lignin residue after enzymatic hydrolysis and organosolv lignin derived from Maplewood were used to measure the kinetic behaviours of lignin pyrolysis and to analyse pyrolysis product distributions. The enzymatic lignin residue pyrolyzed at lower temperature than that of organosolv lignin. The differential thermogravimetric (DTG) peaks for pyrolysis of the enzymatic residue were more similar to the DTG peaks for pyrolysis of the original Maplewood than DTG of the organosolv lignin. The condensable liquid volatile products were collected from a Pyroprobe reactor with a liquid nitrogen trap. The primary monomeric phenolic compounds were guaiacol, syringol, and vanillic acid. However, only 14–36 carbon% of the sample could be detected by GC-MS. Over 60 carbon% of the condensable products were heavy tar molecules that are not detectable by GC-MS. These heavy tar molecules are the primary products from pyrolysis of lignin. Intermediate solid samples were also collected at various pyrolysis temperatures and characterized by elemental analysis, FT-IR, DP-MAS 13C NMR, and TOC. The methoxy groups and ether linkages decreased and the non-protonated aromatic carbon–carbon bonds increased in the solid residues as the pyrolysis temperature increased. The carbon content of the initial lignin feed (derived from enzymatic hydrolysis) and the solid polyaromatics residue (obtained at 773 K) was 58 wt% and 74 wt% respectively. This polyaromatic residue contained about 69 wt% of the original lignin feed. The solid polyaromatics undergo further slow decomposition accompanied by a constant release of carbon dioxide as the pyrolysis reaction continues. The pyrolysis of the enzymatic lignin residue was modelled by two reactions in series. In the first pyrolysis step the lignin was decomposed with an apparent activation energy of 74 kJ mol−1 and a heat of reaction of −8,780 kJ kg−1. The second pyrolysis step had an apparent activation energy of 110 kJ mol−1 and a heat of reaction of −2,819 kJ kg−1. Lignin pyrolysis has lower activation energies and higher heats of reaction than cellulose pyrolysis.

Sequence Analysis of Polyether-Based Thermoplastic Polyurethane Elastomers by 13C NMR

In this Note we would like to report on the analysis of the microstructure of segmented polyurethanes by 13CNMRmaking use of the signals arising from nonprotonated aromatic carbons which appear to be sensitive to dyads sequence distribution. The polyurethanes here studied were obtained from poly(tetramethylene glycol) (PTMG) as macrodiol soft segment, 1,4-butanediol (BD) chain extender, and 4,40'-methylenediphenyl diisocyanate (MDI) as hard segment.The results obtained in this work provide a relatively simple and reliable method to determine the microstructure of MDI-based thermoplastic polyurethanes by means of the NMR technique.

Tryptophan chemical shift in peptides and proteins: a solid state carbon-13 nuclear magnetic resonance spectroscopic and quantum chemical investigation.

We have obtained the carbon-13 nuclear magnetic resonance spectra of a series of tryptophan-containing peptides and model systems, together with their X-ray crystallographic structures, and used quantum chemical methods to predict the (13)C NMR shifts (or shieldings) of all nonprotonated aromatic carbons (C(gamma), C(delta 2) and C(epsilon 2). Overall, there is generally good accord between theory and experiment. The chemical shifts of Trp C(gamma) in several proteins, hen egg white lysozyme, horse myoglobin, horse heart cytochrome c, and four carbonmonoxyhemoglobins, are also well predicted. The overall Trp C(gamma) shift range seen in the peptides and proteins is 11.4 ppm, and individual shifts (or shieldings) are predicted with an rms error of approximately 1.4 ppm (R value = 0.86). Unlike C(alpha) and N(H) chemical shifts, which are primarily a function of the backbone phi,psi torsion angles, the Trp C(gamma) shifts are shown to be correlated with the side-chain torsion angles chi(1) and chi(2) and appear to arise, at least in part, from gamma-gauche interactions with the backbone C' and N(H) atoms. This work helps solve the problem of the chemical shift nonequivalences of nonprotonated aromatic carbons in proteins first identified over 30 years ago and opens up the possibility of using aromatic carbon chemical shift information in structure determination.

Modification of Model Structures of Upper Freeport Coal Extracts Using 13C NMR Chemical Shift Calculations

The extract from carbon disulfide/N-methyl-2-pyrrolidinone mixed solvent extraction of Upper Freeport coal at room temperature was fractionated with acetone and pyridine into three fractions, and solid-state 13C NMR measurements of these fractions were made. Model structures for the three extract fractions constructed by Takanohashi et al. (Energy Fuels 1998, 12, 1168) were estimated using the NMR chemical shift calculation method. By comparing the spectra calculated for the model structures with the observed spectra, some corrections were made to the chemical structures of the original models to fit their spectra. Compared with the observed spectra, for the acetone-soluble fraction (AS), the model structure was richer in nonprotonated aromatic carbon and β methylene carbon, and poorer in protonated aromatic carbon and methyl carbon, and, for the acetone-insoluble−pyridine-soluble fraction (PS) and pyridine-insoluble−mixed-solvent-soluble fraction (PI), the spectra calculated for the model structures showed higher fractions for nonprotonated aromatic carbon and methylene carbon, and lower fractions for methyl carbon. We proposed modified model structures based on NMR chemical shift calculations for the three fractions.