High-resolution Solid-state 13C Research Articles

Molecular structure and evolution mechanism of shale kerogen: Insights from thermal simulation and spectroscopic analysis

During thermal evolution, the kerogen in shale formation undergoes significant chemical, structural and compositional changes, continuously influencing the shale storage capacity, hydrocarbon generation potential, and gas occurrence state. This study systematically examines the chemical structural changes during the thermal evolution of Longmaxi shale kerogen using hydrothermal laboratory experiments from 420 to 850 °C. Additionally, a range of characterization techniques, including Fourier Transform Infrared (FTIR) spectroscopy, Raman spectroscopy, High-Resolution Transmission Electron Microscopy (HRTEM), X-ray Diffraction (XRD), and Solid-state 13C Nuclear Magnetic Resonance (13C NMR), were employed to systematically investigate the kerogen structure and its evolution patterns. Results indicate that Longmaxi Formation kerogen comprises a large molecular carbon framework with aromatic structures, long-chain aliphatic components, and oxygen-containing functional groups. As thermal maturity reaches 2.6 %, the alignment of aromatic fringes gradually increased, with over 60 % aligning in the main direction. Throughout thermal evolution (1.9–3.2 %), fringes in the 5–10 Å range within aromatic structures peak at over 20.45 %, with fringes over 50 Å rapidly increasing from 3 % to over 15 %. Early in the thermal evolution, the long-chain aliphatic component of the kerogen decreases rapidly and part of the structure forms small aromatic rings through aromatisation. The mature to over-mature stage can be divided into three phases: 1) 1.7–2.4 % (loss of oxygen-containing functional groups), 2) 2.4–3.5 % (formation of larger aromatic structures via condensation reactions), 3) over 3.5 % (continued aggregation of aromatic rings, resulting in an increase in the length of aromatic fringes). At a macroscopic level, this induces changes in kerogen pore structure and carbonization features. This study thus provides new insights into the thermal evolution of shale kerogen, which in turn improve understanding of shale reservoirs for hydrocarbon exploitation.

Understanding the Solid-State Structure of Riboflavin through a Multitechnique Approach.

Crystalline riboflavin (vitamin B2) performs an important biological role as an optically functional material in the tapetum lucidum of certain animals, notably lemurs and cats. The tapetum lucidum is a reflecting layer behind the retina, which serves to enhance photon capture and vision in low-light settings. Motivated by the aim of rationalizing its biological role, and given that the structure of biogenic solid-state riboflavin remains unknown, we have used a range of experimental and computational techniques to determine the solid-state structure of synthetic riboflavin. Our multitechnique approach included microcrystal XRD, powder XRD, three-dimensional electron diffraction (3D-ED), high-resolution solid-state 13C NMR spectroscopy, and dispersion-augmented density functional theory (DFT-D) calculations. Although an independent report of the crystal structure of riboflavin was published recently, our structural investigations reported herein provide a different interpretation of the intermolecular hydrogen-bonding arrangement in this material, supported by all the experimental and computational approaches utilized in our study. We also discuss, more generally, potential pitfalls that may arise in applying DFT-D geometry optimization as a bridging step between structure solution and Rietveld refinement in the structure determination of hydrogen-bonded materials from powder XRD data. Finally, we report experimental and computational values for the refractive index of riboflavin, with implications for its optical function.

Mechanochemical synthesis of mechanical bonds in M12L8 poly-[n]-catenanes.

Using mechanochemistry by grinding TPB and ZnBr2, an amorphous poly-[n]-catenane of interlocked M12L8 nanocages is obtained in good yields (∼80%) and within 15 minutes. The mechanical bond among the icosahedral M12L8 cages in the amorphous phase has been demonstrated by single crystal XRD, powder XRD and FT-IR spectroscopy following an amorphous-to-crystalline transformation by guest uptake of the amorphous phase. High-resolution solid-state 13C NMR spectroscopy gives insights into the local structure of the amorphous catenane focusing on TPB aromatic-aromatic interactions.

HIGHLY POROUS HYBRID COMPOSITE HYDROGELS BASED ON CELLULOSE AND 1,10-PHENANTHROCYANINE OF Zn(II): SYNTHESIS AND CHARACTERIZATION WITH WAXS, FTIR, 13C CP/MAS NMR AND SEM

Hybrid composite hydrogels were synthesized by immobilization of 1,10-phenanthrocyanine zinc(II) complex in cellulose hydrogels. The hydrogels exhibited long-term stability, high water retaining capacity and porosity, which were determined using chemical methods. They were characterized with wide-angle X-ray scattering (WAXS), Fourier transform infrared spectroscopy (FTIR), high-resolution solid-state 13C CP/MAS NMR spectroscopy and scanning electron microscopy (SEM). According to FTIR analysis, the immobilization of the complex in the hydrogels led to its interaction with the cellulose inside the matrices of the hydrogels. Additional crystallization of the cellulose in the hydrogels occurred during the formation of the composite hydrogels, as revealed with WAXS. The surface and morphology of the cross-sections of the hydrogels, as well as the pore distribution and pore size, were defined with SEM. The SEM study also ascertained the impact of the molecular mass of the embedded complex on the efficiency of the immobilization.

Evaluation of chitosan crystallinity: A high-resolution solid-state NMR spectroscopy approach

We propose a novel approach relied on high-resolution solid-state 13C NMR spectroscopy to quantify the crystallinity index of chitosans (Ch) prepared with variable average degrees of acetylation (DA¯) from 5% to 60 % and average weight molecular weight (M¯w) ranged in 0.15 × 106 g mol−1–1.2 × 106 g mol−1. The Dipolar Chemical Shift Correlation (DIPSHIFT) curve of the C(6)OH segment revealed increased mobility dynamic, which induced different distribution from trans-to-gauche conformations in relation to C(4). Indeed, 1H-13C Heteronuclear Correlation (2D HETCOR) showed that distinguished C4 chemical shifts correlates with the same aliphatic protons. The short-range ordering can be assigned to C4/C6 signals on 13C CPMAS and, for our case, the deconvolution procedure between disordered and ordered phases revealed increasing crystallinity with DA¯, as confirmed by SVD multivariate analysis. This work extended the knowledge regarding the use of 13C CPMAS technique to predict the crystallinity of chitosans without the use of amorphous standards.

Self-healing UV-curable polymer network with reversible Diels-Alder bonds for applications in ambient conditions

The ambient-temperature self-healing potential of a poly(methacrylate) network, containing reversible furan-maleimide Diels-Alder crosslinks, is studied. A reversible bis(methacrylate) monomer, containing Diels-Alder bonds, is synthesized and characterized by means of 1H NMR spectroscopy. Subsequent polymerization via UV-cure yields a reversible polymer network, consisting of thermoplastic poly(methacrylate), crosslinked by Diels-Alder bonds. Characterization of the polymer network via high-resolution solid-state 13C NMR indicates that undesirable side reactions, such as the formation of irreversible maleimide homo- and copolymers, are prevented by protecting the maleimide functionality in the Diels-Alder adduct. The thermal reversibility of the polymer network is studied by means of Fourier transform infrared spectroscopy and differential scanning calorimetry, and it is confirmed that the reversibility of the Diels-Alder reaction remains during several thermal cycles between −40 °C and 85 °C. Dynamic mechanical analysis confirms that the reversible polymer network maintains sufficient mechanical properties even at elevated temperatures up to 110 °C, at which retro Diels-Alder reaction is favored. Self-healing of the polymer network at ambient temperature, even in the fully vitrified state at 20 °C, is demonstrated by grinding the polymer material into a powder and then healing the powder (after compression) to form a rectangular bar with recovered mechanical properties. Ambient temperature healing without human intervention is therefore feasible, making this material suitable as e.g. encapsulant in photovoltaic modules for outside applications.

<sup>13</sup>C/<sup>7</sup>Li Solid-State Nuclear Magnetic Resonance Study on Poly(Ethylene Glycol)-Poly(Propylene Glycol)-Poly(Ethylene Glycol)/LiCF<sub>3</sub>SO<sub>3</sub> Copolymer Electrolytes

Solid-state high-resolution13C/7Li nuclear magnetic resonance (NMR) study was performed on the phase structure and chain dynamics of PEG-PPG-PEGn/LiCF3SO3(n=3, 6, 12) copolymer electrolytes. PEG repeating units and Li+form PEG3:LiCF3SO3crystalline complex and PE3/Li+amorphous complex in all the samples. PPG repeating units and Li+form different complexes with respect to O:Li+feed ratio (denoted as PP/Li+-3/6/12). The13C chemical shifts and half widths of the signals from PP/Li+-3/6/12 remain unchanged, which implies the structures of PP/Li+-3/6/12 are similar at least in a very short range. The half width of the7Li signals from PP/Li+-3/6/12 becomes narrower and narrower as the Li+concentration decreases. This indicates the chain mobility of the amorphous phase increases with the decrease of ionic concentration. Moreover, neat crystalline PEG, neat amorphous PEG and neat amorphous PPG start to appear when O:Li+is greater than 3:1 and their contents increase with the increase of O:Li+. Overall, solid-state high-resolution NMR is a powerful and unique method for understanding the phase structure and chain dynamics of solid polymer electrolytes (SPEs), more applications of this technique to studies on SPEs is expecting.

Strikingly different molecular organization and molecular order of tetracatenar mesogens in columnar mesophases revealed by XRD and 13C NMR.

For a successful design of functional mesogens, it is paramount to understand factors that contribute to molecular organisation such as molecular shape, the non-covalent interactions of the constituent moieties as well as nanosegregation of incompatible molecular parts. In this study on four tetracatenar mesogens, we show that by a slight change in the length of the terminal chain, the molecular organization changes from lamellar to columnar phase and that the orientational order experiences profound change between the lamellar, the center rectangular columnar and the hexagonal columnar mesophases. We consider here, mesogens that exhibit lamellar and columnar mesophases with five phenyl rings in the central rod-like core which are subjected to XRD and high resolution solid state 13C NMR investigations in their mesophases. The XRD studies indicate that the lower homologs exhibit a lamellar mesophase while the higher homologs show either a centre rectangular columnar phase or a 2D hexagonal columnar mesophase. 13C NMR investigations reveal interesting and strikingly different molecular orientations in each of these phases. For example, values of order parameters of one of the phenyl rings in the core region of the mesogens vary from 0.75 and 0.77 for the lamellar mesogens to 0.45 and 0.17 for the centre rectangular columnar and the hexagonal columnar mesogens respectively. While these values indicate that the mesogenic molecules are oriented along the magnetic field as expected in the lamellar phases, the very low order parameter in the hexagonal columnar phase arises due to molecules distributed azimuthally in layers and undergoing motion about the columnar axis which itself is oriented orthogonal to the magnetic field. Such cutting edge information extracted from the combined use of XRD and 13C NMR studies on tetracatenar mesogens is expected to be of significant use for the study of π-conjugated polycatenar systems where functional properties depend on the molecular orientation and order.

Molecular and Segmental Orientational Order in a Smectic Mesophase of a Thermotropic Ionic Liquid Crystal

We investigate conformational dynamics in the smectic A phase formed by the mesogenic ionic liquid 1-tetradecyl-3-methylimidazolium nitrate. Solid-state high-resolution 13C nuclear magnetic resonance (NMR) spectra are recorded in the sample with the mesophase director aligned in the magnetic field of the NMR spectrometer. The applied NMR method, proton encoded local field spectroscopy, delivers heteronuclear dipolar couplings of each 13C spin to its 1H neighbours. From the analysis of the dipolar couplings, orientational order parameters of the C–H bonds along the hydrocarbon chain were determined. The estimated value of the molecular order parameter S is significantly lower compared to that in smectic phases of conventional non-ionic liquid crystals.

Elucidating the internal structure and dynamics of [formula omitted]chitin by 2DPASS-MAS-NMR and spin-lattice relaxation measurements

The structure and dynamics of the second most abundant biopolymer α−chitin were studied by high resolution solid state 13C cross-polarization magic angle spinning nuclear magnetic resonance (CP-MAS-NMR) spectral analysis, 13C relaxation measurements at eight chemically different carbon sites and chemical shift anisotropy measurement by two-dimensional phase-adjusted spinning sidebands (2DPASS) magic angle spinning (MAS) solid state NMR method.13C spin-lattice relaxation time was measured by high resolution Torchia CP method. Spin-lattice relaxation rate (1/T1) of side chain carbon nuclei were remarkably high, because those nuclei possess higher degree of motional freedom. Chemical shift anisotropy parameters of eight chemically different carbon nuclei were determined by 2DPASS-MAS-NMR experiment. Large value of chemical shift anisotropy was observed for carbonyl group carbon (C7) nuclei, because of electrostatic effect, hydrogen bonding and molecular magnetic susceptibility. 13C relaxation mechanism is mainly governed by chemical shift anisotropy interaction, especially at high value of external magnetic field (11.74 T). Thus, the correlation time at different carbon sites were also calculated by using the spin-lattice relaxation times and chemical shift anisotropy values. The correlation time of side chain carbon (C8) was two orders of magnitude less than the carbonyl group carbon. These types of investigations would enlighten the correlation between the structure and dynamics of long polysaccharide chain compound.

Temperature Dependence of Phase Structure for Highly Drawn Ultrahigh Molecular Weight Polyethylene Using Solid-State High-Resolution 13C Nuclear Magnetic Resonance

Temperature Dependence of Phase Structure for Highly Drawn Ultrahigh Molecular Weight Polyethylene Using Solid-State High-Resolution 13C Nuclear Magnetic Resonance

Cationic Pyridylamido Adsorbate on Brønsted Acidic Sulfated Zirconia: A Molecular Supported Organohafnium Catalyst for Olefin Homo- and Co-Polymerization

Here we report the combined application of high-resolution solid-state 13C–CPMAS-NMR and FT-IR spectroscopy, elemental analysis, kinetic poisoning/active site counting, variable dielectric constant medium, and DFT computation to characterize the surface chemistry of a pyridylamido hafnium complex (Cat1, L1-HfMe2, L1 = 2,6-diisopropyl-N-{(2-isopropylphenyl)[6-(naphthalen-1-yl)pyridin-2-yl]methyl}aniline) adsorbed on Bronsted acidic sulfated zirconia (ZrS). The spectroscopic and DFT results indicate protonolytic formation of organohafnium cations having a largely electrostatic pyridylamido-Hf-CH3+···ZrS– interaction with elongated Hf···OZrS distances of ∼2.14 A. High-molecular-weight polyethylenes and ethylene/1-octene copolymers are obtained with this supported catalyst without an activator/cocatalyst. The DFT calculations reveal that the first ethylene insertion into the Hf-methyl bond has a lower barrier than the corresponding insertion into the Hf-aryl bond of this single-site heterogeneous catalyst.

Remarkably enhanced H2 evolution activity of oxidized graphitic carbon nitride by an extremely facile K2CO3-activation approach

The photocatalytic activity enhancement of graphitic carbon nitride (g-C3N4) in water splitting were commonly achieved by introducing various functional groups onto/into its planar structure. Herein, oxidized g-C3N4 (CNO) was prepared via an extremely facile K2CO3 activation approach. The high-resolution XPS spectra and solid-state 13C NMR spectra confirmed the successful introduction of O-containing groups and O atoms were inherited the original molecular framework of g-C3N4, but exhibited effectively enlarged bonded with C atoms rather than N atoms in the tri-s-triazine structure of g-C3N4. CNO inherited the original molecular framework of g-C3N4, but exhibited effectively enlarged surface area and enhanced visible light absorption, and more attrractively, the band structure of g-C3N4 could be well-tuned with the introduction of oxygen species. Importantly, the surface O-containing moieties could strongly interact with cocatalyst Pt and thus remarkably promote interfacial electron transfer and consequent photo-generated charge carrier separation. Consequently, the as-obtained CNO exhibited the remarkably enhanced photocatalytic performance with a H2 evolution rate of 199.7 μmol h−1, which is 9 times that of g-C3N4 (22.2 μmol h−1).

β-Cyclodextrin-epichlorohydrin polymer/graphene oxide nanocomposite: preparation and characterization

In this study, we report a novel and simple technique to synthesize water-insoluble nanocomposite macromolecule composed of graphene oxide (GO) nanosheets and β-cyclodextrin (β-CD) polymer. These nanocomposites were synthesized by the one-pot precipitation polymerization process via polycondensation of β-CD with epichlorohydrin (ECH) in an aqueous suspension of GO under alkaline condition. The obtained β-CD polymer/GO (CDP-ECH/GO) were characterized by thermal gravimetry, differential scanning calorimetry, infrared and Raman spectroscopies, high-resolution solid state 13C NMR, X-ray diffraction, electron microscopy (both SEM and TEM), and energy dispersive spectroscopy. Furthermore, the mechanism of CDP-ECH/GO nanocomposites formation is also discussed depending on the type of mixing device that has been used during polymerization. The swelling ratio of the nanocomposites as well as their absorption properties toward methylene blue (MB), phenol (PN), 1-naphtol (1-NPH) and 2-naphtol (2-NPH) have been determined. Increasing the GO content in the nanocomposites results in an increase of the adsorption capacity of the materials.

Chain packing in the noncrystalline region of deuterated UHMWPE: A solid-state 2H and 13C NMR study

Chain packing in the non-crystalline region of a semi-crystalline polymer is strongly influenced by the crystallization conditions. Using a single-site catalytic system, deuterated Ultra High Molecular Weight Polyethylenes (UHMW-PEs) are synthesized and the chain topology of the noncrystalline region is investigated by solid-state NMR. The variability in chain packing within the noncrystalline region is introduced on crystallizing the same polymer either by controlled synthesis or solution or melt. 2H NMR combined with line shape simulations yield detailed information on the geometry and frequency of the segmental motion in the noncrystalline regions of deuterated UHMWPE crystallized at three different conditions. High resolution solid-state 13C NMR reveals the temperature dependence of the conformation statistics of chain segments in the noncrystalline regions of the samples. Combining the observations from the 2H and 13C NMR, the influence of segmental motions on the conformation statistics in the noncrystalline regions of the samples are discussed.

Solid-State NMR and the Crystallization of Aspartic and Glutamic Acids

We used high-resolution solid-state 13C NMR, with cross-polarization and magic-angle spinning, to study chirality in the crystallization of aspartic acid. We show that, contrary to a recent report, dl-aspartic acid crystallizes over most of its temperature range as racemic crystals rather than a conglomerate of enantiomeric crystals, regardless of whether the solution was prepared by solution of the racemate or by mixing solutions of the pure enantiomers. Over virtually the entire solution temperature range at 1 bar pressure, racemic crystals of aspartic acid are thermodynamically stable, whereas the conglomerate is metastable. In contrast, in agreement with the literature, glutamic acid crystallizes as a conglomerate under thermodynamic conditions. Solid-state NMR results are confirmed by powder X-ray diffraction.

Exploiting Powder X-ray Diffraction to Establish the Solvent-Assisted Solid-State Supramolecular Assembly of Pillar[5]quinone

We report the solvent-mediated supramolecular assembly of pillar[5]quinone (P[5]Q), a symmetric cyclamer containing five benzoquinone moieties bridged by five alternating methylene units. The supramolecular assembly of P[5]Q is shown to be facilitated by 1,1,2,2-tetrachloroethane (TCE) as solvent, producing a microcrystalline solvate material P[5]Q·2TCE with a fluffy texture. Optical and electron microscopy reveal that this material has a rod-shaped morphology, extending to several micrometers in length. Due to the microcrystalline nature of the material, structure determination was carried out directly from powder X-ray diffraction data, augmented by high-resolution solid-state 13C NMR. The two crystallographically distinct TCE molecules occupy different types of void in the structure and have different dynamic properties. Crystallization of P[5]Q was attempted from a large number of different solvents, but only TCE was found to facilitate the formation of a crystalline phase. Indeed, features of the crystal structure suggest that the TCE component plays an important role in promoting the columnar assembly of P[5]Q molecules.

Distribution ratio of carbon black in polyisobutyrene/polyisoprene rubber blends using high-resolution solid-state 13C NMR

We establish the 13C nuclear magnetic resonance (NMR) method as the best available method for the evaluation of carbon black (CB) distribution in rubber blend. We find that the line widths of NMR peaks increase as CB contents increase. Using this phenomenon it is found that, in polyisobutyrene rubber (IIR)/polyisoprene rubber (IR)/CB composites, CB more easily distributes in IR than in IIR. And we can estimate that more than 70% of CB is distributed in to the IR phase, irrespective of CB content.

Bound solvent in different stereoregular syndiotactic polypropylene gels

The coagulation size of the solvent in syndiotactic polypropylene (sPP)/o-dichlorobenzene gels was investigated for different stereoregular samples with racemic diads of 98% (sPP98) and 83% (sPP83) using thermal analysis, optical microscopy, and solid-state high resolution 13C nuclear magnetic resonance (NMR). For the sPP98 gel, the coagulation size of freezable bound solvent was smaller than that in the sPP83 gel. The non-crystalline region of the sPP83 gel is large due to the incomplete stereoregularity, so that a large coagulation of the solvent occurs. The weight contents of the crystalline, amorphous, and interphase components were estimated using solid-state high resolution 13C NMR. The interphase weight content was comparable with that of the non-freezable solvent estimated by thermal analysis, which implies that the non-freezable solvent is bound in the interphase. The coagulation size of the solvent in the sPP98 gel increased with gelation temperature. Large spherulites are formed at higher gelation temperatures; therefore, the non-crystalline region between lamellae is larger, which results in the large coagulation size of the solvent.

Preparation and characterization of sol–gel-modified pineapple leaf fiber/polylactic acid composites

In this study, the fibers generated from agricultural waste-pineapple leaf are obtained through exposure, drying, crushing and sifting. A novel sol–gel method is utilized to modify the pineapple leaf fiber (PALF), which is subsequently treated by a coupling agent. The aim is to improve the compatibility between PALF and the polymer matrix and to enhance the heat resistance and mechanical properties of the composite material. Furthermore, a series of modified PALF/polylactic acid (PLA) composites are prepared. FTIR, high resolution solid-state 13C and 29Si NMR experiments show that the PALF was successfully modified by the silane coupling agent and sol–gel method. Polarizing optical microscopy analysis reveals PLA crystal growth of a sufficiently high density along the polymer-fiber interface. Moreover, the storage and loss moduli of PLA are increased by adding the modified PALF. Apart from the enhancement of the mechanical properties, the incorporation of modified PALF reduces the amount of agricultural waste and extends the application of PLA.