13C Magic Angle Spinning Research Articles

Graphite Oxide-TiO2 Nanocomposite Type Photocatalyst for Methanol Photocatalytic Reforming Reaction

Graphite-oxide/TiO2 (GO/TiO2) composite materials were prepared by heterocoagulation method from Brodie’s graphite-oxide (GO) in order to test them as catalysts in the methanol photocatalytic reforming reaction in liquid phase. The preparation of the composite itself resulted in only little changes in the structure of GO as it was indicated by attenuated total reflection infrared (ATR-IR) and 13C magic-angle spinning nuclear magnetic resonance (13C MAS NMR) spectroscopic measurements. However, during the photocatalytic reaction, all of the GO/TiO2 samples darkened strongly indicating structural changes of GO. X-ray photoelectron spectroscopy along with NMR confirmed the loss of oxygen functionalities and emergence of graphitic species in the samples recovered from the photocatalytic reaction. Model experiments were designed to identify the key factors determining the activity of the GO/TiO2 derived photocatalysts. It was found that the emergence of a pronounced coupling between TiO2 and the graphite-like carbonaceous material is the most important contribution to get active and stable photocatalysts.

Polymeric Selenium Sulfides As Promising Cathode Materials for High-Energy Lithium Batteries

Lithium/sulfur (Li/S) batteries have been widely studied for high-energy applications during the past few decades owing to the high theoretical specific capacity (1675 mAh g-1), low cost, natural abundance and environmentally benign nature of sulfur.1 However, the unceasing dissolution/diffusion (shuttle effects) of reaction intermediates (polysulfides, S x 2-, 4 ≤ x ≤ 8) during battery operation and the insulating nature of sulfur lead to low Coulombic efficiency (CE), short cycle life and poor rate capability, critically limiting the commercial deployment of Li/S batteries. To address these issues, extensive efforts have been conducted by utilizing physical or chemical confinement of S as well as polysulfides within conductive hosts.2 Recently, chemically stable S-rich copolymers were reported as novel active materials for Li/S batteries, exhibiting effective suppression of the shuttle effects of polysulfides within cathodes.3, 4 However, the poor electrical conductivity of polymeric materials leads to lower utilization of sulfur in copolymer matrix as well as unsatisfactory rate capability of Li/S batteries.5 Selenium (Se), as a congener of S, has been reported as an alternative cathode for Li batteries due to its higher electronic conductivity (Se: 1 × 10−3 S m−1 vs. S: 5 × 10−28 S m−1) and comparable theoretical volumetric capacity (Se: 3,253 Ah L−1­ vs. S: 3,467 Ah L−1) than sulfur.6 Herein, to combine the high conductivity of Se and excellent confinement capability of S-rich copolymers, we prepared polymeric selenium-sulfides, utilizing chemical bonds between S, Se and C atoms, by the inverse vulcanization method with S, SeS2 and 1,3-diisopropenylbenzene (DIB) as comonomers (noted as poly(S-SeS­2-DIB)). We first investigated the effects of chemical incorporation of Se in S-rich copolymers on the cycling performance of Li/S batteries. To further enhance the electrochemical utilization and rate capability of poly(S-SeS­2-DIB), porous carbon (KetjenBlack600, KB600) network was utilized as the conductive host within copolymers. Among all the copolymer composite samples that were systematically investigated, the poly(S-SeS­2-DIB) with an optimal S/SeS2 mass ratio of 9:1 exhibited the highest initial specific capacity (1218 mAh g−1 at 100 mA g−1) and superior rate capability (537 mAh g−1 at 2000 mA g−1). The novel molecular structures of poly(S-SeS­2-DIB) composites were revealed by 13C cross polarization and magic angle spinning nuclear magnetic resonance spectra as well as X-ray photoelectron spectroscopy. The effects of SeS2 contents in poly(S-SeS­2-DIB) on the electrochemical properties of Li/S batteries such as rate capability and CE were further analyzed by combined electrochemical and microstructural characterization methods. Figure 1. (a) The molecular structures of DIB, S copolymer (noted as PS10) and S-SeS2 copolymer (noted as PSS90-10); (b) Cycling performances of PS10, PSS90-10 and PSS90-10@KB600 cathodes under the current density of 500 mA g–1; (c) Rate capabilities of poly(S-SeS2-DIB) composites with different contents of SeS2 under various current densities. Reference: Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Nat Mater 2011, 11, (1), 19-29.Fang, R.; Zhao, S.; Sun, Z.; Wang, D. W.; Cheng, H. M.; Li, F. Adv Mater 2017.Chung, W. J.; Griebel, J. J.; Kim, E. T.; Yoon, H.; Simmonds, A. G.; Ji, H. J.; Dirlam, P. T.; Glass, R. S.; Wie, J. J.; Nguyen, N. A.; Guralnick, B. W.; Park, J.; Somogyi, A.; Theato, P.; Mackay, M. E.; Sung, Y. E.; Char, K.; Pyun, J. Nat Chem 2013, 5, (6), 518-24.Hu, G.; Sun, Z.; Shi, C.; Fang, R.; Chen, J.; Hou, P.; Liu, C.; Cheng, H. M.; Li, F. Adv Mater 2016.Simmonds, A. G.; Griebel, J. J.; Park, J.; Kim, K. R.; Chung, W. J.; Oleshko, V. P.; Kim, J.; Kim, E. T.; Glass, R. S.; Soles, C. L.; Sung, Y.-E.; Char, K.; Pyun, J. ACS Macro Letters 2014, 3, (3), 229-232.Abouimrane, A.; Dambournet, D.; Chapman, K. W.; Chupas, P. J.; Weng, W.; Amine, K. J Am Chem Soc 2012, 134, (10), 4505-8. Figure 1

Silane-based hyper-cross-linked porous polymers and their applications in gas storage and water treatment

A series of novel silane-based hyper-cross-linked porous polymers was prepared through the Friedel–Crafts alkylation reaction of dimethyldiphenylsilane as the monomer and formaldehyde dimethyl acetal as the external cross-linker. The structures of the polymers were confirmed by FTIR and solid-state cross-polarization magic-angle spinning 13C NMR. We investigated the porous polymers with different ratios of dimethyldiphenylsilane to formaldehyde dimethyl acetal. Porous properties were determined by nitrogen, carbon dioxide and hydrogen adsorptions. The results revealed that the porous materials possessed surface areas from 742 to 1205 m2 g−1 with the increasing amount of formaldehyde dimethyl acetal. Moreover, this material exhibited high thermal stabilities, good CO2 capture, and H2 storage capacities. The material was also used as adsorbent for organic dyes in aqueous solution and showed an excellent adsorption capacity of 952 mg g−1 for Congo red dye. These results suggest that the silane-based hyper-cross-linked porous polymers are promising materials for CO2 capture, H2 storage, and water treatment.

Role of Salts in Phase Transformation of Clathrate Hydrates under Brine Environments

Although ion exclusion is a naturally occurring and commonly observed phenomenon in clathrate hydrates, an understanding for the effect of salt ions on the stability of clathrate hydrates is still unclear. Here we report the first observation of phase transformation of structure I and structure II clathrate hydrates using solid-state 13C, 19F, and 23Na magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy, combined with X-ray diffraction and Raman spectroscopy. The phase transformation of clathrate hydrates in salt environments is found to be closely associated with the quadruple point of clathrate hydrate/hydrated salts and the eutectic point of ice/hydrated salts. The formation of the quasi-brine layer (QBL) is triggered at temperatures a little lower than the eutectic point, where an increasing salinity and QBL does not affect the stability of clathrate hydrates. However, at temperatures above the eutectic point, all hydrated salts and the QBL melt completely to form brine solutions, desta...

Facile silane functionalization of graphene oxide.

The facile silane functionalization of graphene oxide (GO) was achieved yielding vinyltrimethoxysilane-reduced graphene oxide (VTMOS-rGO) nanospheres located in the inter-layer spacing between rGO sheets via an acid-base reaction using aqueous media. The successful grafting of the silane agent with pendant vinyl groups to rGO was confirmed by a combination of Fourier-transform infrared (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The structure and speciation of the silane-graphene network (nanosphere) and, the presence of free vinyl groups was verified from solid-state magic angle spinning (MAS) and solution 13C and 29Si nuclear magnetic resonance (NMR) measurements. Evidence from Scanning Electron Microscopy (SEM), High-Resolution Transmission Electron Microscopy (HRTEM) and TEM-High-Angle Annular Dark-Field (TEM-HAADF) imaging showed that these silane networks aided the exfoliation of the rGO layers preventing agglomeration, the interlayer spacing increased by 10 Å. The thermal stability (TGA/DTA) of VTMOS-rGO was significantly improved relative to GO, displaying just one degradation process for the silane network some 300 °C higher than either VTMOS or GO alone. The reduction of GO to VTMOS-rGO induced sp2 hybridization and enhanced the electrical conductivity of GO by 105 S m-1.

The deactivation of a ZnO doped ZrO2-SiO2 catalyst in the conversion of ethanol/acetaldehyde to 1,3-butadiene.

A deactivation study on the ethanol/acetaldehyde conversion to 1,3-butadiene over a ZnO promoted ZrO2–SiO2 catalyst prepared by a sol–gel method was performed. The samples were characterized by N2 adsorption–desorption isotherms, scanning electron microscopy (SEM), NH3 temperature programmed desorption (NH3-TPD), X-ray powder diffraction characterization (XRD), thermogravimetric analyses (TGA), Fourier transform infrared resonance (FT-IR), 13C magic-angle spinning nuclear magnetic resonance (13C NMR) and X-ray photoelectron spectroscopy (XPS). The pore structure characteristics and surface acidity of Zn0.5–Zr–Si catalysts were largely decreased with time-on-stream and no crystal structure was formed in the used catalyst, indicating that the deactivation was caused by carbon deposition. Two main types of carbon deposition were formed, namely low-temperature carbon deposition with the oxidation temperature of around 400 °C and high-temperature carbon deposition with the oxidation temperature of 526 °C. The carbon species were mainly composed of graphitized carbon, amorphous carbon, carbon in C–O bonds and carbonyls. The deactivated catalyst could be regenerated by a simple oxidation process in air. Adding a certain amount of water into the feed had a positive effect on reducing the carbon deposition.

Preparation of cyclic carbonate via cycloaddition of CO2 on epoxide using amine-functionalized SAPO-34 as catalyst

Amine-functionalized chabazite (CHA) type silicoaluminophosphate (SAPO-34) materials were prepared under hydrothermal conditions by a direct co-condensation method. Different concentrations of an organo-amine (3-aminopropyltrimethoxysilane) were incorporated into the CHA-type framework under hydrothermal conditions. The crystalline structures of the organoamine modified SAPO-34 materials with different concentrations of amine functionalities were confirmed by powder XRD. The morphology of the materials was confirmed by SEM, FE-SEM, and TEM analysis. The successful incorporation of amine-functionalities into the wall of SAPO-34 framework was confirmed by 29Si and 13C magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The acidity and basicity of the materials was studied by NH3 and CO2 temperature programmed desorption (TPD). The resultant amine-functionalized SAPO-34 materials were utilized for the cyclo-addition of CO2 to epoxide in liquid phase medium. The organoamine-modified SAPO-34 acted as a bi-functional (acidic and basic) catalyst. The incorporated amine sites played a crucial role in CO2 activation on the surface, while the acidic sites of the SAPO-34 framework accelerated the epoxide ring opening. Thus, the amine-functionalized SAPO-34 prepared as described in this work can be an effective catalyst for the activation and utilization of CO2 in cyclic carbonate synthesis from epoxides (98%) conversion of epichlorohydrin with 96% selectivity toward cyclic carbonate.

Structure of melanins from the fungi Ochroconis lascauxensis and Ochroconis anomala contaminating rock art in the Lascaux Cave

Two novel species of the fungal genus Ochroconis, O. lascauxensis and O. anomala have been isolated from the walls of the Lascaux Cave, France. The interest in these fungi and their melanins lies in the formation of black stains on the walls and rock art which threatens the integrity of the paintings. Here we report solid-state cross polarization magic-angle spinning 13C and 15N nuclear magnetic resonance (NMR) spectroscopy and surface-enhanced Raman spectroscopy (SERS) of the melanins extracted from the mycelia of O. lascauxensis and O. anomala in order to known their chemical structure. The melanins from these two species were compared with those from other fungi. The melanins from the Ochroconis species have similar SERS and 13C and 15N NMR spectra. Their chemical structures as suggested by the data are not related to 3,4-dihydroxyphenylalanine, 5,6-dihydroxyindole or 1,8-dihydroxynaphthalene precursors and likely the building blocks from the melanins have to be based on other phenols that react with the N-terminal amino acid of proteins. The analytical pyrolysis of the acid hydrolysed melanin from O. lascauxensis supports this assumption.

Moving towards fast characterization of polymorphic drugs by solid-state NMR spectroscopy

Solid-state nuclear magnetic resonance (SS-NMR) spectroscopy has become a common technique to study polymorphism in pharmaceutical solids at high-resolution. However, high-throughput application of high resolution SS-NMR spectroscopy is severely limited by the long 1H spin-lattice relaxation (T1) that is common to solid phase compounds. Here, we demonstrate the use of paramagnetic relaxation reagents such as chromium (III) acetylacetonate (Cr(acac)3) and nickel (II) acetylacetonate (Ni(acac)2) for fast data acquisition by significantly reducing the T1 value for carbamazepine Forms I, II, III, and dihydrate, cimetidine Forms A and B, nabumetone Form I, and acetaminophen Form I polymorphs. High resolution 13C cross-polarization and magic angle spinning were used to measure T1 values for each polymorph. In order to confirm the absence of polymorphic transitions during SS-NMR experiments, powder x-ray diffraction was implemented. The amount of chromium ions incorporated by the recrystallization process was quantified by using inductively coupled plasma optical emission spectroscopy. Our results suggest that the paramagnetic ions added to the polymorphs do not affect the polymorphic transformation or the quality of NMR spectra. We believe that this successful demonstration of fast data collection will enable high-throughput utilization of SS-NMR techniques to study polymorphic solids and could set the groundwork for NMR crystallography studies.

Characterization and Cu sorption properties of humic acid from the decomposition of rice straw.

Humic acid (HA) derived from rice straw decomposed for 1 (HA-1), 3 (HA-3), 6 (HA-6) and 12 (HA-12) months was characterized by potentiometric titration and solid-state cross-polarization magic-angle spinning 13C nuclear magnetic resonance spectroscopy (CPMAS 13C NMR). The sorption of Cu on examined HA was investigated using a combination of batch sorption, isothermal titration calorimetry (ITC) and sequential desorption. Results showed that the functional group content and the humification degree of HA tended to increase with increasing decomposition time especially in the latter stage of examined decomposition period. Cu sorption on HA was a rapid process that occurred within the first 1h and the sorption capacity increased from 245.4mmolkg-1 on HA-1 to 294.6mmolkg-1 on HA-12. The sorption of Cu was endothermic, spontaneous and the randomness was increased during Cu sorption. Sorbed Cu on examined HA can be hardly released by NH4Ac but nearly fully released by EDTA. Forming inner-sphere complexes was the main mechanism of Cu sorption on examined HA. This study could provide valuable information for a better understanding on the environmental impacts of the decomposition of organic waste.

Evident variations of fungal and actinobacterial cellulolytic communities associated with different humified particle-size fractions in a long-term fertilizer experiment

Cellulose is the dominant form of carbon (C) existing in arable soils, however the ecology of its degradation in soil is still relatively poorly understood. Here, community abundance and composition of fungal and actinobacterial cellulolytic genes (cbhI and GH48) from glycoside hydrolase family 7 and 48 together with characterization of fulvic acid (FA) and humic acid (HA) determined by cross polarization magic angle spinning (CPMAS) 13C nuclear magnetic resonance (NMR) spectroscopy were explored in five soil particle-size fractions (large macroaggregate, coarse sand, fine sand, silt and clay), collected from a 33-yr mineral and organic fertilizer experiment. The results revealed the significant effects of particle-size fraction and fertilization on the distribution of soil humus and cellulolytic microbial community abundance. Strong correlations were detected between C content and structure of soil humus with cellulolytic microbial abundance. Generally, larger fractions (>63 μm) especially fine sand, which showed a lower degree of humification with higher aromaticity, lower HA/FA ratio, aliphaticity and alkyl/O-alkyl ratio of HA, were associated with greater abundance of cellulolytic microbes. However, smaller fractions (<63 μm), especially the clay fraction, showed lower cbhI and GH48 gene abundances with a greater degree of humification indicated by 13C NMR spectra. Phylogenetic analysis of the obtained nucleotide sequences revealed undiscovered sequences of both fungal and actinobacterial cellulolytic microbes. However, no clear clustering of sequences from particular particle-size fraction or fertilizer treatment was observed, even though combined application of chemical fertilizer and manure significantly increased cellulolytic gene abundances.

Structural phase transitions and ferroelastic properties of perovskite-type layered (CH3NH3)2CdCl4

Herein, we perform a structural characterization of (CH3NH3)2CdCl4 by 1H and 13C magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectrometry, discussing the geometry around CH3NH3 cations. 1H MAS NMR determined two sets of protons in hydrogen-bonded CH3NH3 groups, exhibiting large T1ρ (L) and small T1ρ (S) values corresponding to long and short C-H/N-H bonds, respectively. The spin-lattice relaxation time of 113Cd in CdCl6 octahedra is shown to exhibit an anomaly near 283 K (TC2) due to the occurrence of a phase transition that is not governed by changes in the motion of CH3NH3 groups but is accompanied by the corresponding changes for Cd in CdCl6 units. The observed domain pattern corresponds to an exchange of a- and c-axes of the orthorhombic structure, with the resulting domain orientations viewed as the main reason for the occurrence of phase transitions, and changes in CdCl6 group motion observed by 113Cd NMR.

Solid state deuterium NMR study of LKα14 peptide aggregation in biosilica.

In nature, organisms including diatoms, radiolaria, and marine sponges use proteins, long chain polyamines, and other organic molecules to regulate the assembly of complex silica-based structures. Here, the authors investigate structural features of small peptides, designed to mimic the silicifying activities of larger proteins found in natural systems. LKα14 (Ac-LKKLLKLLKKLLKL-C), an amphiphilic lysine/leucine repeat peptide with an α-helical secondary structure at polar/apolar interfaces, coprecipitates with silica to form nanospheres. Previous 13C magic angle spinning studies suggest that the tetrameric peptide bundles that LKα14 is known to form in solution may persist in the silica-complexed form, and may also function as catalysts and templates for silica formation. To further investigate LKα14 aggregation in silica, deuterium solid-state nuclear magnetic resonance (2H ssNMR) was used to establish how leucine side-chain dynamics differ in solid LKα14 peptides isolated from aqueous solution, from phosphate-buffered solution, and in the silica-precipitated states. Modeling the 2H ssNMR line shapes probed the mechanisms of peptide preaggregation and silica coprecipitation. The resulting NMR data indicates that the peptide bundles in silica preserve the hydrophobic interior that they display in the hydrated solid state. However, NMR data also indicate free motion of the leucine residues in silica, a condition that may result from structural deformation of the aggregates arising from interactions between the surface lysine side chains and the surrounding silica matrix.

Multi-scale instrumental analyses for structural changes in steam-treated bamboo using a combination of several solid-state NMR methods

Steam-treated bamboo was analyzed for structural changes at multiple scales using several solid-state NMR methods, including relaxation time analysis. Subtle changes due to different processing conditions were analyzed by 13C magic angle spinning (MAS) NMR using cross polarization (CP), single pulse with dipolar decoupling (DD), and pulse saturation transfer (PST). The analyses also used 1H spin-lattice relaxation time (T1H) integrated with attenuated total reflection infrared spectroscopy (ATR-IR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The first change due to the steam treatment appeared in the hemicellulose methyl groups and the aromatic carboxylic acid; with further processing the carbohydrate ring of hemicellulose decreased and the pyrolysis products having quaternary olefinic and aromatic groups increased. During steam treatment, pyrolysis products appear to have restrained molecular motions of the steam-heated bamboo, while cleavage of lignin increased molecular motion of the OCH3 group. Bamboo treated with pressurized steam retained its hemicellulose and produced a smaller amount of pyrolysis products, while both hemicellulose and lignin were reduced by treatment with NaOH solution. The physical fracture of the sample observed at higher processing temperatures and longer times occurred not because of chemical changes in specific constituents but through physical destruction of vascular bundle units.

Facile Method for Fluorescent Labeling of Starch Nanocrystal

Fluorescently labeled starch nanocrystal (FL-SNC) was synthesized using a simple, low-cost, and scalable two-step chemical modification process. Reactive amino groups were introduced onto the SNC surface through silanization with 3-aminopropyl triethoxysilane (APTES), and fluorescein isothiocyanate (FITC) groups were covalently attached through thiourea. Fourier transform infrared spectrometry, X-ray photoelectron spectroscopy, solid-state cross-polarization magic-angle spinning 13C NMR, UV–visible absorbance spectrophotometry, and fluorescence emission spectroscopy confirmed the successful introduction of the fluorescent groups. Transmission electron microscopy and X-ray diffraction data indicated that the dispersibility of FL-SNC was significantly improved, and the original crystallinity and morphology were retained. Compared with a mixture of uncoupled FITC and SNC, covalently connected FL-SNC displayed a more obvious fluorescence intensity and higher photostability. Furthermore, FL-SNC was biocompatib...

Solid-State NMR Study of Dynamic Properties of a Columnar Liquid Crystalline Macrocycle

Abstract A shape-persistent macrocycle composed of carbazole and salphen, which has a discrete 1.4-nm-sized inner cavity, forms four thermotropic columnar liquid crystalline (LC) phases. Both the structure and dynamics of the macrocyclic mesogens in the LC phases were investigated by solid-state NMR. Variable temperature 1H and 13C magic-angle spinning (MAS) experiments reveal that the peripheral side chains exhibited narrow line widths throughout the columnar phases, indicating that the side chains behave like liquids among the columns. In sharp contrast, the NMR signals related to the macrocyclic mesogen became highly symmetric and similar to signals in solution with increasing temperature. These results revealed that the LC macrocycle with an inherent inner cavity gains more mobility around the mesogenic framework as the macrocycle underwent phase transitions.

Non-destructive and direct determination of the degree of substitution of carboxymethyl cellulose by HR-MAS 13C NMR spectroscopy

We report on the direct assessment of the degree of substitution (DS) of carboxymethyl cellulose (CMC) by High Resolution Magic Angle Spinning (HR-MAS) 13C NMR spectroscopy. The method is applied to industrial CMCs with low and high viscosity and nominal DS, purified and technical samples, and from cellulose linters or wood. The preparation of a set of purified CMC working standards with accurate DS values for the method validation is also described. The DS values determined via HR-MAS 13C NMR on the industrial samples are critically compared to the corresponding values achieved through the USP 37 〈281〉 method (ASH method) and the HPLC method, and the advantages and limitations of the HR-MAS NMR method highlighted. Finally, the HR-MAS NMR approach allowed the accurate DS assessment in CMC with low DS, characterized by a non-negligible fraction of non-functionalized cellulose. The proposed “effective DS” accounts for the DS of the solvent-exposed CMC.

Solid-state NMR studies for the development of non-woven biomaterials based on silk fibroin and polyurethane

Silk fibroin (SF) possesses several characteristics that are favorable for tissue engineering. However, the mechanical properties must be modified in some cases. For instance, soft tissues such as those of the circulatory system are more elastic than tissue-engineered materials. The development of polymer blends is a simple method with the potential to provide materials with extended useful properties. In this study, we developed a non-woven SF and thermoplastic polyurethane-blend sheet from a polymer solution. The structure and miscibility of the blend sheet was evaluated using solid-state nuclear magnetic resonance (NMR) methods. In the 13C cross-polarization and magic angle spinning NMR spectra, no peak shift was observed between the pure and blended samples. The miscibility of the sample was investigated using proton spin-lattice relaxation times in the laboratory frame (T1H). The T1H measurement clearly revealed that the molecular chains of SF and Pellethane exist in close proximity of several tens of nanometers. The development of polymer blends is a simple method with the potential to provide materials with extended useful properties. Here, the SF and Pellethane were observed to be highly miscible with each other even though SF maintained the strong β-sheet crystal structure. This result suggests that SF/Pellethane-blended sheets possess strength that is derived from SF and elasticity that is derived from Pellethane. Therefore, this blended sheet should be a good medical material for soft tissue engineering.

Surface Interactions and Confinement of Methane: A High Pressure Magic Angle Spinning NMR and Computational Chemistry Study.

Characterization and modeling of the molecular-level behavior of simple hydrocarbon gases, such as methane, in the presence of both nonporous and nanoporous mineral matrices allows for predictive understanding of important processes in engineered and natural systems. In this study, changes in local electromagnetic environments of the carbon atoms in methane under conditions of high pressure (up to 130 bar) and moderate temperature (up to 346 K) were observed with 13C magic-angle spinning (MAS) NMR spectroscopy while the methane gas was mixed with two model solid substrates: a fumed nonporous, 12 nm particle size silica and a mesoporous silica with 200 nm particle size and 4 nm average pore diameter. Examination of the interactions between methane and the silica systems over temperatures and pressures that include the supercritical regime was allowed by a novel high pressure MAS sample containment system, which provided high resolution spectra collected under in situ conditions. For pure methane, no significant thermal effects were found for the observed 13C chemical shifts at all pressures studied here (28.2, 32.6, 56.4, 65.1, 112.7, and 130.3 bar). However, the 13C chemical shifts of resonances arising from confined methane changed slightly with changes in temperature in mixtures with mesoporous silica. The chemical shift values of 13C nuclides in methane change measurably as a function of pressure both in the pure state and in mixtures with both silica matrices, with a more pronounced shift when meso-porous silica is present. Molecular-level simulations utilizing GCMC, MD, and DFT confirm qualitatively that the experimentally measured changes are attributed to interactions of methane with the hydroxylated silica surfaces as well as densification of methane within nanopores and on pore surfaces.

Humic acid composition and humification processes in wetland soils of a Mediterranean semiarid wetland

The present study focuses on a compositional characterization of the humic acid (HA) fraction of several wetland soils using solid-state 13C NMR spectroscopy. The data were analysed using the molecular mixing model (MMM), based on an empirical approach by Nelson and Baldock. The compositional data from HAs obtained with this model were used to obtain a wider assessment of the process of humification from comparison of total soil wetland organic matter composition and HA composition. Twenty samples of humic acids (HAs) isolated from a Mediterranean semiarid wetland (‘Tablas de Daimiel’, central Spain) were studied using elemental analysis and cross polarization magic angle spinning (CPMAS)13C nuclear magnetic resonance (NMR) spectroscopy. The NMR data were analysed with the molecular mixing model (MMM) considering up to six generic components (carbohydrate, protein, lignin, lipid, char and ‘carbonyl’). HAs are considered a conceptual mixture of these model components, and the MMM determines the proportions of the characteristic biomolecules contributing to HA composition. The composition of the HAs under study depends on local factors such as site vegetation and occurrence of fire. Correlations between the proportions of the six generic components and further comparison with those determined for the unfractionated OM (whole sample, WS), gave information on HA origin and humification mechanisms. In particular, the proportions of char and carbohydrate (R 2 0.637) and contents of lignin and protein (R 2 0.471) in the HAs were negatively correlated (P < 0.05). Significant correlations (R 2 0.439) also existed for char contents in whole sample (WS) compared to HA, and for carbohydrates in WS compared to HA (R 2 0.558). Char proportion grew in HA with respect to the WS, and carbohydrates dropped to a half on average in HA compared to WS. Two different humification mechanisms could be identified for no-fire and fire areas. In the former, HA-char was preserved selectively from char in the sample, whereas in the latter, char was newly formed by fire effect.