Solid-state 13C Cross-polarization Magic-angle Spinning Research Articles

Trash into treasure: Converting waste polyester into C3N4-based intramolecular donor-acceptor conjugated copolymer for efficient visible-light photocatalysis

Carbon nitride (C3N4) is regarded as one of the most prospective photocatalysts for environmental remediation. The design of intramolecular donor-acceptor (D-A) conjugated structure has aroused particular interest for improving visible-light photocatalytic activity of C3N4, but remains a huge challenge. Herein, a novel C3N4-based D-A conjugated copolymer is synthesized by incorporating aromatic ring into the C3N4 skeleton by copolymerization of melamine with terephthalic acid from the in-situ controlled degradation of recycled poly(ethylene terephthalate) catalyzed by ZnO at 360 °C. The results of X-ray diffraction, solid-state 13C cross-polarization magic-angle spinning nuclear magnetic resonance, thermodynamics analysis, and X-ray photoelectron spectroscopy confirm the structure of C3N4-based D-A conjugated system, consisting of terminal amino group of the tri-s-triazine unit as electron-donating group and aromatic ring as acceptor. Notably, the C3N4-based D-A conjugated copolymer displays shorter lifetime of light-induced charge carriers, lower photoluminescence emission, and larger photocurrent response than pristine C3N4, certifying that the D-A conjugated system significantly promotes the separation efficiency of light-induced charge carriers. As such, the C3N4-based D-A conjugated copolymer displays high performance in the photocatalytic degradation of Amido black 10B upon visible light illumination, of which the kinetic constant is 101 times that of unmodified C3N4. Lastly, reactive oxygen species including ·OH, ·O2- and 1O2 are proved to be the main active sites. This work not only offers new insights into the synthesis of efficient C3N4-based D-A conjugated copolymers, but also will inspire new paradigms toward sustainable transformation of waste plastics into value-added products for environmental remediation.

Physical aspects of the biopolymer matrix in wheat bran and its dissected layers

The objectives of this work were to 1) determine the physical structure of untreated wheat bran and the differences in physical structure between its dissected layers; 2) evaluate how bran hydration affected bran crystallinity and polymer order; and 3) determine how enzymatic treatment of wheat bran affected its physical structure. For the first time, X-ray diffraction (XRD), small angle X-ray scattering (SAXS), solid-state 13C cross-polarization magic-angle spinning nuclear magnetic resonance (13C CP/MAS NMR), and polarized light microscopy with a waveplate were used to study the physical structure of wheat bran and its dissected layers. The XRD and solid-state 13C CP/MAS NMR both confirmed the presence of crystalline cellulose in untreated bran, enzymatically treated bran, and dissected bran layers. The outer pericarp had the highest crystallinity of the dissected bran layers and showed negative birefringence. The aleurone layer was low in cellulose content and completely amorphous, yet the cell walls in the aleurone layer showed strong positive birefringence. The treatment of destarched and deproteinated bran with the Updegraff reagent removed amorphous material, leaving its crystalline cellulose structure intact. Hydration of the outer pericarp increased its crystallinity index and CP/MAS NMR resonance intensity, which indicated a possible increase in polymer order. The SAXS also confirmed that cell wall polymers, possibly aggregated cellulose microfibrils, increased in order as a result of hydration.

Dynamic Covalent Synthesis of Crystalline Porous Graphitic Frameworks

Summary Porous graphitic framework (PGF) is a two-dimensional (2D) material that has emerging energy applications. An archetype contains stacked 2D layers, the structure of which features a fully annulated aromatic skeleton with embedded heteroatoms and periodic pores. Due to the lack of a rational approach in establishing in-plane order under mild synthetic conditions, the structural integrity of PGF has remained elusive and ultimately limited its material performance. Here, we report the discovery of the unusual dynamic character of the C=N bonds in the aromatic pyrazine ring system under basic aqueous conditions, which enables the successful synthesis of a crystalline porous nitrogenous graphitic framework with remarkable in-plane order, as evidenced by powder X-ray diffraction studies and direct visualization using high-resolution transmission electron microscopy. The crystalline framework displays superior performance as a cathode material for lithium-ion batteries, outperforming the amorphous counterparts in terms of capacity and cycle stability.

Synthesis of Nitrogen-Rich Polymers by Click Polymerization Reaction and Gas Sorption Property.

Microporous organic polymers (MOPs) are promising materials for gas sorption because of their intrinsic and permanent porosity, designable framework, and low density. The introduction of nitrogen-rich building block in MOPs will greatly enhance the gas sorption capacity. Here, we report the synthesis of MOPs from the 2,4,6-tris(4-ethynylphenyl)-1,3,5-triazine unit and aromatic azides linkers by click polymerization reaction. Fourier transform infrared (FTIR) and solid-state 13C CP-MAS (Cross Polarization-Magic Angle Spinning) NMR confirm the formation of the polymers. CMOP-1 and CMOP-2 exhibit microporous networks with a BET (Brunauer–Emmett–Teller) surface area of 431 m2·g−1 and 406 m2·g−1 and a narrow pore size distribution under 1.2 nm. Gas sorption isotherms including CO2 and H2 were measured. CMOP-1 stores a superior CO2 level of 1.85 mmol·g−1 at 273 K/1.0 bar, and an H2 uptake of up to 2.94 mmol·g−1 at 77 K/1.0 bar, while CMOP-2, with its smaller surface area, shows a lower CO2 adsorption capacity of 1.64 mmol·g−1 and an H2 uptake of 2.48 mmol·g−1. In addition, I2 vapor adsorption was tested at 353 K. CMOP-1 shows a higher gravimetric load of 160 wt%. Despite the moderate surface area, the CMOPs display excellent sorption ability for CO2 and I2 due to the nitrogen-rich content in the polymers.

Accessing Quantitative Degrees of Functionalization on Solid Substrates via Solid-State NMR Spectroscopy

The development of a solid-state nuclear magnetic resonance (NMR) method allowing the quantification of active sites (i.e., residual vinyl groups) accessible for chemical functionalization on the surface of poly(divinyl benzene) microspheres is presented. Residual vinyl groups of poly(divinyl benzene) microspheres (PDVB55 and PDVB80) were quantified via solid-state 13C cross-polarization magic-angle spinning (CP-MAS) NMR spectroscopy. In addition, 13C CP-MAS NMR spectroscopy allows the comparison of core and grafted microspheres functionalization on the same (arbitrary) scale in a short measuring time. This scale was calibrated by an extended absolute quantification of the vinyl groups using 13C single pulse excitation (SPE) MAS NMR spectroscopy. The degree of cross-linking of the microspheres was calculated to be 30 and 50% for PDVB55 and PDVB80 microspheres, respectively. The number of active groups per nominal surface area is 110 and 179 groups per nm2 for PDVB55 and PDVB80 microspheres, respectively. The loading capacities of the microspheres (e.g., 0.61 and 0.65 mmol·g−1) are not too far removed from those found in Merrifield resins of comparable sizes.

Cyanide- and carboxylate-functionalized cubic mesoporous silicas SBA-1: Synthesis, characterization and reactivity of organic functional groups

Highly ordered cubic ( Pm 3 ¯ n ) mesoporous silicas SBA-1 functionalized with a high density of cyanide and carboxylate groups were synthesized by one-pot co-condensation of tetramethoxysilane (TMOS) and 2-cyanoethyltriethoxysilane (CNTES) using cetyltriethylammonium bromide (CTEABr) as the structure-directing agent under strongly acidic conditions. A high degree of structural ordering is still retained up to 25 mol% CNTES loading (based on silicon) in the synthesis mixture. The simultaneous existence of chemically attached cyanide and carboxylate ester moieties was evidenced by solid-state 13C CPMAS (cross polarization magic angle spinning) NMR. The relative concentrations between cyanide and carboxylate ester functional groups in the resulting mesoporous silicas are strongly dependent on the synthesis parameters such as acid type, acid concentration and reaction time. By controlling these synthesis parameters, the contents of the cyanide functional groups incorporated can be tuned, and their transformation into carboxylate ester groups can be followed. With increasing HCl concentration and reaction time, the cyanide functional groups incorporated convert progressively into carboxylate ester groups after solvent extraction with an HCl/ethanol mixture. While using H 2SO 4 as an acid medium, on the other hand, the cyanide functional groups can be totally transformed into carboxylate ester groups. The generation of carboxylic acid functionalities can be achieved either by solvent extraction with only ethanol (without HCl) or post-dealkylation treatment of carboxylate ester groups in concentrated HCl. The acid type and concentration and the silicon source also have a profound influence on the particle morphology of mesoporous silicas.

Carbon-13 CP MAS NMR Study on Structures of Octadecyl Chains Influenced by Co-Presence of 3-Aminopropyl Chains on SBA-15

Functionalized SAB-15 samples by octadecyltrimethoxysilane (OTC) were studied by 13C magic angle spinning (MAS) cross polarization (CP) nuclear magnetic resonance (NMR) spectroscopy. In the SBA-15 sample fully functionalized by 3-aminopropyltrimethoxysilane (APS) and OTC in 1:1 molar ratio, octadecyl chains were observed to have, on average, more trans conformation than those in the SBA-15 samples fully modified by OTC only. Our results confirm that long chain molecules tend to organize themselves better in the co-presence of short chain molecules on the surface of mesoporous materials by packing of the different length chains in an interdigitized fashion even when the short chains are long enough to have three carbons and a functional group at the ends. In addition, our results indicate that solid-state 13C CP MAS NMR spectroscopy is a simple and non-destructive method to probe the molecular structures of the domains composed of long alkyl chains.

Chemical and carbon-13 cross-polarization magic-angle spinning nuclear magnetic resonance characterization of logyard fines from British Columbia.

Phasing out beehive burners and rising costs for landfilling have increased the need to widen options for utilization of the smaller size fractions of woody wastes generated during log handling and sawmilling in British Columbia. We characterized several size classes of logyard fines up to 16 mm sampled from coastal and interior operations. Total C, total N, ash, and condensed tannin concentrations were consistent with properties derived largely from wood, with varying proportions of bark and mixing with mineral soil. Especially for < 3-mm fractions, the latter resulted in high ash contents that would make them unsuitable for fuel. Solid-state 13C cross-polarization magic-angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectra were consistent with the chemical data, with high O-alkyl intensity and similarity to naturally occurring woody forest floor; no samples were high in aromatic or phenolic C. Aqueous extracts of two < 16-mm fines, which accounted for only a small proportion of the total C, were enriched in alkyl C and had low or undetectable tannins. Application to forest sites might cause short-term immobilization of N, but also might include possible longer-term benefits from reduction of N loss after harvesting and restoration of soil organic matter in degraded sites.

Controlled synthesis of stable poly(vinyl formamide- co -vinyl amine)/silica hybrid particles by interfacial post-cross-linking reactions

The chemical synthesis and the physicochemical properties of stable poly(vinyl formamide-co-vinyl amine)/silica hybrid particles are presented. Copolymers of poly(vinyl formamide) (PVFA) and poly(vinyl amine) (PVAm) and their protonated forms were adsorbed onto silica from aqueous solutions. The influences of the pH strength and the ion concentration of the aqueous solution as well as the copolymer composition (degree of hydrolyzation of PVFA), and the molecular mass on the adsorption process were investigated by electrokinetic measurements, potentiometric titration, and quantitative elemental analyses. Silica surface-charge neutralization is achieved at a pH strength above 10 for highly hydrolyzed (95%) PVFA polymers. Decreasing the amino content in the PVAm chain shifts successively both the point of zero charge and the isoelectric point to lower pH values. PVFA-co-PVAm layers onto silica are adsorbed weakly. To fix these layers irreversibly, cross-linking reactions with (4,4′-diisocyanate)diphenyl methane were carried out on the surface of solid PVFA-co-PVAm/silica hybrid particles suspended in acetone. The cross-linking reaction, which is connected with the conversion of amino groups, is also a tool to control the surface charge of the PVFA-co-PVAm/silica hybrids. X-ray photoelectron spectroscopy and solid-state 13C cross-polarization magic-angle spinning NMR spectroscopy were used to obtain information on the number of and the structure of the functionalized polyelectrolyte layers on silica. The success of cross-linking was also shown by the results of these spectroscopic methods.

Solid-state nuclear magnetic resonance relaxation times in crosslinked macroporous polymer particles of divinylbenzene homopolymers

Monodisperse porous particles of poly(divinylbenzene) prepared by the activated swelling method have been investigated by solid-state 13C crosspolarization magic-angle spinning (CPMAS) nuclear magnetic resonance (NMR) relaxation measurements. Homopolymeric combinations of two porogens (toluene and 2-ethylhexanoic acid) and two monomers (meta- and para-divinylbenzene) were studied. Residual vinyl groups were systematically reacted with increasing amounts of bromine, producing 20 different polymers samples for which we measured crosspolarization times, TCH, proton rotating frame spin-lattice relaxation, T, 13C spin-lattice relaxation, T, and proton spin-lattice relaxation, T. These parameters were chosen to reflect expected changes in a wide range of frequencies of motion as a function of structure. Relative differences in the molecular mobility of the major functional groups (aromatic, vinyl and aliphatic) is related to initial reactants used, vinyl concentration, relative reactivity of vinyl groups, distribution of vinyl groups, pore structure, and degree of crosslinking. Variable temperature 1H combined rotation and multiple pulse NMR (CRAMPS) was used to derive activation energies for selected samples via measurement of the proton spin-lattice relaxation time, T. Irreversible thermal effects were observed in ambient temperature relaxation after heating to temperatures in the range of 393–418 K. Simple univariate statistical analyses failed to reveal consistent correlations among the known variables. However, the application of more sophisticated multivariate and neural network analyses allowed excellent structure–property predictions to be made from the relaxation time data. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1307–1328, 1999

Zeise's salt studied by high resolution solid state 13C CPMAS and 1H SPEDA NMR spectroscopy

Polycrystalline Zeise's salt ( K(CH 2 CH 2 PtCl 3⊎x H 2 O ) was investigated by high resolution solid state 13C CPMAS (cross polarization magic-angle spinning) and 1H SPEDA (single pulse excitation with delayed acquisation) NMR spectroscopies using different rotor spinning speeds. Though the 13C and 1H nuclei are respectively two and three bond lengths separated from the 35 37 Cl nuclei, both the 13C and 1H spectral lineshapes are influenced by the second and higher order dipolar couplings between 13C and 1H and the quadrupole spins 35 37 Cl . An extra splitting or broadening resulting from the coupling between 13C and 195Pt is also found. The high resolution 1H spectrum is also influenced by 1H 195Pt coupling. A computer program was developed to simulate theoretically the observed 13C and 1H NMR spectral lineshapes.

13C CP-MAS NMR study of compatibility in blends of poly(3-octyl-thiophene) and poly(phenylene oxide)

Blends of undoped poly(3-octyl thiophene) and poly(phenylene oxide) have been investigated using solid-state 13C cross-polarization magic-angle spinning (CP-MAS) NMR. Proton spin-lattice relaxation times in the rotating frame (T 1ϱ(H)) are unchanged in the blends as compared with the homopolymers. This shows that the blends are incompatible on a molecular level. The averaging of laboratory frame spin-lattice relaxation times (T 1(H)) indicated that the maximum domain size is approximately 100 nm for POT-concentrations below 30%.