Solid-state CP/MAS C-NMR analysis of cellulose and tri-O-substituted cellulose ethers

Highly crystalline tri-O-substituted cellulose ethers having ethyl, n-propyl, n-butyl, allyl, and methallyl substituents were prepared from low-molecular weight cellulose (DP = 15). Influences of conformational and packing effects on solid-state ¹³C-NMR spectra were studied by using X-ray diffraction and solid- and solution-state ¹³C-NMR analyses of the cellulose derivatives. Unit-cell sizes tentatively obtained from X-ray diffraction patterns of the cellulose derivatives indicated that conformations and packing states of cellulose chains and alkyl chains of substituents were different between the derivatives. Solid- and solution-state ¹³C-NMR spectra of cellulose allomorphs, and effects of hydrogen bonds present in celluloses I, II, and III on chemical shifts of their solid-state ¹³C-NMR spectra were proposed.     

TEMPO-mediated oxidation of native cellulose: Microscopic analysis of fibrous fractions in the oxidized products

The 2,2,6,6-tetramethylpiperidine-1-oxy radial (TEMPO)-mediated oxidation was applied to aqueous suspensions of cotton linters, ramie and spruce holocellulose at pH 10.5, and water-insoluble fractions of the TEMPO-oxidized celluloses collected by filtration with water were analyzed by optical and transmission electron microscopy and others. The results showed that both fibrous forms and microfibrillar nature of the original native celluloses were maintained after the TEMPO-mediated oxidation, even though carboxylate and aldehyde groups of 0.67–1.16 and 0.09–0.21 mmol/g, respectively, were introduced into the water-insoluble fractions. Neither crystallinity nor crystal size of cellulose I of the original native celluloses was changed under the conditions adopted in this study. Carboxylate groups in the TEMPO-oxidized ramie were mapped by labeling with lead ions as their counter ions. The transmission electron micrographs indicated that some heterogeneous distribution of carboxylate groups along each cellulose microfibril or each bundle of cellulose microfibrils seemed to be present in the TEMPO-oxidized celluloses.     

Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose

Never-dried and once-dried hardwood celluloses were oxidized by a 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated system, and highly crystalline and individualized cellulose nanofibers, dispersed in water, were prepared by mechanical treatment of the oxidized cellulose/water slurries. When carboxylate contents formed from the primary hydroxyl groups of the celluloses reached approximately 1.5 mmol/g, the oxidized cellulose/water slurries were mostly converted to transparent and highly viscous dispersions by mechanical treatment. Transmission electron microscopic observation showed that the dispersions consisted of individualized cellulose nanofibers 3-4 nm in width and a few microns in length. No intrinsic differences between never-dried and once-dried celluloses were found for preparing the dispersion, as long as carboxylate contents in the TEMPO-oxidized celluloses reached approximately 1.5 mmol/g. Changes in viscosity of the dispersions during the mechanical treatment corresponded with those in the dispersed states of the cellulose nanofibers in water.     

NMR analyses of polysaccharide derivatives containing amine groups

Amylose, amylopectin, hydroxyethylcellulose, methylcellulose, and cellulose were reacted with diethylaminoethyl chloride HCl salt and 3-chloro-2-hydroxy-propyltrimethylammonium chloride under aqueous alkaline conditions in order to introduce tertiary amine and quaternary ammonium groups into polysaccharides. Degrees of substitution were obtained from ¹H- or ¹³C-NMR spectra of hydrolyzates, and distributions of diethylaminoethyl groups in polysaccharides were measured by ¹³C-NMR. Since amylose, amylopectin, and hydroxyethylcellulose were soluble in the reaction media, these three polysaccharides had higher reactivity for etherifications than cellulose. Methyl-cellulose, which has hydrophobic methyl groups, had as much reactivity as cellulose. Primary hydroxyl groups, C-6, of polysaccharides had the highest reactivity for diethylaminoethylation.     

Comparative study of samples of powdered and microcrystalline celluloses of different natural origins: Supermolecular structure and the chemical composition of powdered samples

Methods of wide-angle X-ray scattering (WAXS), high resolution solid-state ¹³C NMR, and Fourier transform IR-spectroscopy are applied to study supermolecular structures and functional compositions of lignocellulose samples of wood and grass origins and powdered celluloses (PC) obtained from them under identical hydrolysis conditions. It was shown by WAXS that the structure of cellulose I is preserved in samples of powdered celluloses, however, an increased degree of crystallinity and cross-section sizes of crystallites are observed in PC samples. Specific features of changes in the supermolecular structure of cellulose occurred after the hydrolysis, i.e., an increase in the content of cellulose I_β in PC compared to the initial samples, are established by ¹³C NMR method. It was shown by means of ¹³C NMR and Fourier transform IR-spectroscopy that the functional chemical composition of lignocelluloses is weakly affected by the hydrolysis. The presence of residual lignin functional groups in the samples is confirmed.     

Chitin nanocrystals prepared by TEMPO-mediated oxidation of alpha-chitin

Chitin nanocrystals dispersed in water were successfully prepared by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) mediated oxidation of alpha-chitin in water at pH 10 under specific conditions, followed by ultrasonic treatment. When the amount of NaClO added as co-oxidant in the oxidation was 5.0 mmol/g of chitin, the weight percentage of the water-insoluble fraction in the TEMPO-oxidized chitin was 90%, and its carboxylate content reached 0.48 mmol/g. Since the TEMPO-oxidized chitin thus prepared had a crystallinity as high as that of the original alpha-chitin, the C6 carboxylate groups formed by TEMPO-mediated oxidation can be regarded as being present only on the chitin crystallite surfaces. No N-deacetylation occurred on the TEMPO-oxidized chitins. When the TEMPO-oxidized chitin was subjected to ultrasonic treatment in water, mostly individualized chitin nanocrystals were obtained, and the average nanocrystal length and width were 340 and 8 nm, respectively.     

Enhancement of the nanofibrillation of wood cellulose through sequential periodate-chlorite oxidation

Sequential regioselective periodate-chlorite oxidation was employed as a new and efficient pretreatment to enhance the nanofibrillation of hardwood cellulose pulp through homogenization. The oxidized celluloses with carboxyl contents ranging from 0.38 to 1.75 mmol/g could nanofibrillate to highly viscous and transparent gels with yields of 100-85% without clogging the homogenizer (one to four passes). On the basis of field-emission scanning electron microscopy images, the nanofibrils obtained were of typical widths of approximately 25 ± 6 nm. All of the nanofibrillar samples maintained their cellulose I crystalline structure according to wide-angle X-ray diffraction results, and the crystallinity index was approximately 40% for all samples.     

Effect of solvent exchange on the supramolecular structure, the molecular mobility and the dissolution behavior of cellulose in LiCl/DMAc

We investigated the effect of solvent exchange on the supramolecular structure and the molecular mobility of the cellulose molecule to clarify the mechanism of the dissolution of cellulose in lithium chloride/N,N-dimethylacetamide (LiCl/DMAc). Among the celluloses that were solvent exchanged in different ways, the DMAc-treated celluloses dissolved most rapidly. Dissolution of the acetone-treated celluloses was much slower than the DMAc-treated ones, but considerably faster than the untreated one. Such differences in the dissolution behavior were well explained by the differences in the surface fractal dimension calculated from the small-angle X-ray scattering profiles and in the (1)H spin-lattice and spin-spin relaxation times estimated from the solid-state NMR spectroscopic measurements. Furthermore, it was suggested from the IR spectra and the (13)C spin-lattice relaxation times of cellulose that DMAc is adsorbed on the surface of cellulose even after vacuum-drying and affects the molecular mobility and hydrogen-bonding state of cellulose.     

Relationship between length and degree of polymerization of TEMPO-oxidized cellulose nanofibrils

The influence of 2,2,6,6-tetrametylpiperidine-1-oxyl (TEMPO)-mediated oxidation of wood cellulose and the mechanical disintegration of oxidized cellulose in water on degree of polymerization determined by viscosity measurement (DP(v)) and the apparent length of the TEMPO-oxidized cellulose nanofibrils (TOCNs) was investigated. DP(v) values decreased from 1270 to 500-600 with increasing addition of NaClO in the TEMPO-mediated oxidation stage. The DP(v) values were further decreased by mechanical fibrillation in water. There is a linear relationship between the average fibril length and DP(v); the lengths of TOCNs can be approximated from DP(v) using 0.5 M copper ethylenediamine as a solvent of both the cellulose and oxidized celluloses in TOCNs. Based on the cellulose fibril models and TEMPO oxidation mechanism, the depolymerization behavior of TOCNs is tentatively explained in terms of distribution of disordered regions in wood cellulose fibrils and formation of C6-aldehydes in cellulose fibrils during TEMPO-mediated oxidation.     

TEMPO-mediated oxidation of (1 → 3)-β-d-glucans

The 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation was applied to water-insoluble (1 → 3)-β-d-glucans, paramylon and curdlan, to prepare water-soluble oxidized products. When the addition level of NaClO used as the primary oxidant was 15 mmol per gram of the polysaccharide in the combination with catalytic amounts of TEMPO and NaBr under aqueous conditions at pH 10, water-soluble TEMPO-oxidized products were obtained quantitatively within 100 min. ¹³C NMR analysis of the TEMPO-oxidized products revealed that the C6 primary hydroxyl groups of both paramylon and curdlan were completely converted to carboxylate groups by the oxidation. Thus, new (1 → 3)-β-d-polyglucuronic acid sodium salts having almost homogeneous chemical structures can be obtained. The highly crystalline paramylon took longer time for the complete oxidation of the C6 primary hydroxyls to carboxylate groups than curdlan. However, remarkable depolymerization occurs on main chains during the oxidation, and the degrees of polymerization of the water-soluble TEMPO-oxidized products prepared from paramylon and curdlan were only 68 and 86, respectively.     

The surface structure of well-ordered native cellulose fibrils in contact with water

CP/MAS ¹³C NMR spectroscopy was used in combination with spectral fitting to examine the surface structure of hydrated cellulose I fibrils from Halocynthia and Gluconoacetobacter xylinus. To increase the spectral intensities and minimize signal overlap, G. xylinus celluloses site-specifically enriched in ¹³C either on C4 or on both C1 and C6 were examined. The experimental data showed multiple C4 and C6 signals for the water accessible fibril surfaces in the highly crystalline celluloses. These signal multiplicities were attributed to structural features in the surface layers induced by the fibril interior, and could not be extracted by spectral fitting in celluloses with a lower degree of crystallinity such as cellulose from cotton.     

Structure of Co2(CO)6(dppm) and Co2(CO)5(CHCO2Et)(dppm) (dppm = Ph2PCH2PPh2) and exchange reaction with CO: An experimental and computational study

Crystal structures of Co₂(CO)₆(dppm) (1) and Co₂(CO)₅(CHCO₂Et)(dppm) (2) (dppm = Ph₂PCH₂PPh₂) show asymmetry with respect to the orientation of the phenyl groups in 1 and owing to the bridging ethoxycarbonylcarbene ligand in 2. The effect of this asymmetry was recognized in the solid-state ³¹P NMR spectra of 1 and 2 and in the solid-state and solution ¹³C NMR spectra of 2 as well, but not in the solid-state and solution ¹³C NMR spectra of 1. In CH₂Cl₂ solution under an atmosphere of ¹³CO, the CO ligands of both complexes exchange with ¹³CO. The overall rate of ¹³CO exchange at 10 °C was found to be k_obs = 0.107 × 10⁻³ s⁻¹ for 1 and k_obs = 0.243 × 10⁻³ s⁻¹ for 2. Two-layered ONIOM(B3LYP/6-31G(d):LSDA/LANL2MB) studies revealed fluxional behavior of 1 with rather small barriers of activation of the rearrangements. Four possible isomers have been computed for 2, close to each other energetically.     

The confromational behaviour of the cardiac glycoside digoxin as indicated by NMR spectroscopy and molecular dynamics calculations

The ¹H- and ¹³C-NMR spectra of dogoxin in solution in Me₂So-d₆ have been assigned completely. Measurement of the ³J_C,H values has enabled estimation of the torsional angles involving the bonds linking the digitoxose residues, between the inner digitoxose and the genin unit, and for the unsaturated γ-lactone ring. These values have been supplemented by ¹H---¹H NOE data. In general, there is good agreement between the conformations in solution (NMR data) and the solid state (X-ray data), and that derived from theological modelling which shows evidence of conformational flexibility. The major difference occurs for the torsion between the genin and the innermost digitoxose residue where molecular dynamics predict the presence of two conformations, one similar to that seen by NMR and the other similar to the X-ray structure.     

Solid-State 13C Nuclear Magnetic Resonance Characterization of Cellulose in the Cell Walls of Arabidopsis thaliana Leaves

Solid-state 13C nuclear magnetic resonance was used to characterize the molecular ordering of cellulose in a cell-wall preparation containing mostly primary walls obtained from the leaves of Arabidopsis thaliana. Proton and 13C spin relaxation time constants showed that the cellulose was in a crystalline rather than a paracrystalline state or amorphous state. Cellulose chains were distributed between the interiors (40%) and surfaces (60%) of crystallites, which is consistent with crystallite cross-sectional dimensions of about 3 nm. Digital resolution enhancement revealed signals indicative of triclinic and monoclinic crystalline forms of cellulose mixed in similar proportions. Of the five nuclear spin relaxation processes used, proton rotating-frame relaxation provided the clearest distinction between cellulose and other cell-wall components for purposes of editing solid-state 13C nuclear magnetic resonance spectra.     

WAXS and 13C NMR study of Gluconoacetobacter xylinus cellulose in composites with tamarind xyloglucan

- Model composites, produced using cellulose from stationary cultures of the bacterium Gluconoacetobacter xylinus and tamarind xyloglucan, were examined by wide-angle X-ray scattering (WAXS) and CP/MAS solid-state (13)C NMR spectroscopy. The dominant crystallite allomorph of cellulose produced in culture media with or without xyloglucan was cellulose I(alpha) (triclinic). The presence of xyloglucan in the culture medium reduced the cross-section dimensions of the cellulose crystallites, but did not affect the crystallite allomorph. However, when the composites were refluxed in buffer, the proportion of cellulose I(beta) allomorph increased relative to that of cellulose I(alpha). In contrast, cellulose I(alpha) remained the dominant form when cellulose, produced in the absence of xyloglucan, was then heated in the buffer. Hence the presence of xyloglucan has a profound effect on the formation of the cellulose crystallites by G. xylinus.     

TEMPO-mediated oxidation of β-chitin to prepare individual nanofibrils

TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation was applied to β-chitins, originating from tubeworm and squid pen, to prepare chitin nanofibrils. Water-insoluble fractions of the TEMPO-oxidized β-chitins were obtained in various ratios by controlling the addition level of NaClO used as the primary oxidant in the oxidation. The water-insoluble fractions of the TEMPO-oxidized tubeworm β-chitins, when they had carboxylate groups of 0.18–0.25 mmol/g, were successfully converted to highly viscous and translucent gels by disintegration in water. The gels consisted of mostly individual nanofibrils 20–50 nm in width and at least several microns in length. On the other hand, the water-insoluble fractions of the TEMPO-oxidized squid pen β-chitins could be transformed to neither transparent nor translucent dispersions under any oxidation or disintegration conditions used. When sufficient amounts of NaClO were used, both β-chitins were completely oxidized to the corresponding water-soluble polyuronic acids by oxidation of all C6 primary hydroxyls to carboxylate groups.     

Factors influencing the regeneration of cellulose I from phosphoric acid

The regeneration of cellulose I from phosphoric acid solution was studied, with emphasis both on the conditions required for the regeneration of cellulose I and characterization of the resulting cellulose by X-ray diffraction. Raman spectroscopy and SS¹³C n.m.r. As the conditions of regeneration were varied with respect to temperature and time a variety of polymorphs were produced. As previously reported, the cellulose I polymorph dominated at high temperature and long regeneration time. The question of the authenticity of the regenerated cellulose I was addressed, with several tests confirming that it was not an insoluble residue. Further analysis of the various regenerated celluloses revealed that they all had a small cellulose I component that could be isolated by acid hydrolysis. It is suggested that during regeneration, nuclei of different cellulose polymorphs are formed simultaneously, the proportion of each dependent upon the relative rates of nucleation. Under degradative regeneration conditions the polymorphs more susceptible to hydrolysis are attacked preferentially, leaving behind resistant cellulose I     

Structural changes in microcrystalline cellulose in subcritical water treatment

Subcritical water is a high potential green chemical for the hydrolysis of cellulose. In this study microcrystalline cellulose was treated in subcritical water to study structural changes of the cellulose residues. The alterations in particle size and appearance were studied by scanning electron microscopy (SEM) and those in the degree of polymerization (DP) and molar mass distributions by gel permeation chromatography (GPC). Further, changes in crystallinity and crystallite dimensions were quantified by wide-angle X-ray scattering and (13)C solid-state NMR. The results showed that the crystallinity remained practically unchanged throughout the treatment, whereas the size of the remaining cellulose crystallites increased. Microcrystalline cellulose underwent significant depolymerization in subcritical water. However, depolymerization leveled off at a relatively high degree of polymerization. The molar mass distributions of the residues showed a bimodal form. We infer that cellulose gets dissolved in subcritical water only after extensive depolymerization.     

Solid-state 13C NMR study of na-cellulose complexes

The interaction of microcrystalline cellulose from cotton and aqueous sodium hydroxide was investigated by 13C NMR solid-state spectroscopy as a function of temperature and sodium hydroxide concentration. When the concentration of NaOH was increased, the initial cellulose spectrum was replaced successively by that of Na-cellulose I followed by that of Na-cellulose II. In Na-cellulose I, each carbon atom occurred as a singlet, thus implying that one glucosyl moiety was the independent magnetic residue in the structure of this allomorph. In addition, the occurrence of the C6 near 62 ppm is an indication of a gt conformation for the hydroxymethyl group of Na-cellulose I. In Na-cellulose II, the analysis of the resonances of C1 and C6 points toward a structure based on a cellotriosyl moiety as the independent magnetic residue, in agreement with the established X-ray analysis that has shown that for this allomorph, the fiber repeat was also that of a cellotriosyl residue. For Na-cellulose II, the occurrence of the C6 in the 60 ppm region indicates an overall gg conformation for the hydroxymethyl groups. A comparison of the spectra recorded at 268 K and at room temperature confirms the stronger interaction of NaOH with cellulose when the temperature is lowered. In the Q region, corresponding to NaOH concentrations of around 9% and temperatures below 277 K, most of the sample was dissolved and no specific solid-state 13C NMR spectrum could be recorded, except for that of a small fraction of undissolved cellulose I. The same experiment run on a wood pulp sample leads to a new spectrum, with spectral characteristics different from those of Na-cellulose I and Na-cellulose II. This new spectrum is assigned to the Q phase, which appears to result from topological constraints that are present in whole wood pulp fibers but not in microcrystalline cellulose. A spectrum recorded for samples in the Na-cellulose III conditions resembled that of Na-cellulose II but of lower resolution. Similarly, a spectrum of a sample of Na-cellulose IV was identical to that of hydrated cellulose II. These observations have allowed us to propose a simplified phase diagram of the cellulose/NaOH system in terms of temperature and NaOH concentration. This diagram, which is simpler than the one deduced from X-ray analysis, consists of only four different regions partially overlapping.     

Preparation of cellulose derived from corn stalk and its application for cadmium ion adsorption from aqueous solution

Cellulose was isolated from corn stalk and modified by graft copolymerization to produce an absorbent material (AGCS-cell), which was characterized by scanning electron microscope and energy disperse spectroscopy (SEM-EDS), X-ray diffraction (XRD) and solid-state CP/MAS (13)C NMR. The results showed that AGCS-cell had better adsorption potential for cadmium ion than unmodified cellulose because of the addition of functional groups (CN and OH groups) and the lower crystallinity. The Langmuir isotherms gave the best fit to the data and gave an adsorption capacity was 21.37mgg(-1), which was close to unpurified cellulose (AGCS) and reflected the feasibility of using AGCS-cell as an adsorbent to remove cadmium ions.