Gerhard Zuckerstätter

Effect of autohydrolysis of Eucalyptus globulus wood on lignin structure. Part 1: Comparison of different lignin fractions formed during water prehydrolysis

Abstract The effect of autohydrolysis of Eucalyptus globulus wood was studied with regard to conditions applied in a prehydrolysis-kraft process on the physico-chemical properties of lignin obtained in both the wood residue and hydrolysate. As a reference, milled wood lignin (MWL) was isolated from native wood and compared to three lignin fractions formed during prehydrolysis: 1) lignin from the wood residue isolated as MWL, 2) lignin precipitated from the prehydrolysate during cooling and separated by centrifugation, and 3) lignin degradation products soluble in the prehydrolysate extracted with ethylacetate. All lignin fractions were subjected to Fourier transform infrared (FTIR) spectroscopy, methoxy group determination, elemental analysis, size exclusion chromatography and quantitative nuclear magnetic resonance (NMR) spectroscopy. The results indicate that extensive lignin degradation occurs during prehydrolysis through homolytic cleavage of the aryl-ether bonds resulting in a substantial molecular weight loss of the residual lignin in the treated wood and in the lignin fractions isolated from the prehydrolysate. The aryl-ether cleavage is coupled with a strong increase in phenolic hydroxyl groups and a decrease in aliphatic hydroxyl groups. Indication for condensation reactions were found by NMR spectroscopy.

Effect of autohydrolysis of Eucalyptus globulus wood on lignin structure.Part 2: Influence of autohydrolysis intensity

Abstract Three different levels of autohydrolysis intensity, expressed as the prehydrolysis (P)-factor, were applied to Eucalyptus globulus wood at a liquor/wood ratio of 5:1. Lignin fractions were isolated from the wood residue as milled wood lignin (MWL), from the hydrolysate by centrifugation (insoluble fraction) and by ethyl acetate extraction (soluble fraction), and from the reactor wall as precipitate. With increasing autohydrolysis duration, a decrease in the content of aliphatic hydroxyl groups and of β-O-4 structures was detected in all lignin fractions, whereas the content of phenolic hydroxyl groups increased in the same order. MWL isolated from wood residue after autohydrolysis at the highest P-factor contained only half the β-O-4 structures contained in native lignin. Molecular weight distribution measurements revealed that fragmentation reactions dominated over condensation reactions in all lignin samples investigated. However, low-molecular-weight lignin dissolved in autohydrolysate exhibited extremely high reactivity towards acid-catalysed condensation reaction, which inevitably leads to the formation of sticky precipitates during storage at elevated temperature under the acid conditions prevailing.

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.

Dialkyl Phosphate-Related Ionic Liquids as Selective Solvents for Xylan

Herein we describe a possibility of selective dissolution of xylan, the most important type of hemicellulose, from Eucalyptus globulus kraft pulp using ionic liquids (ILs). On the basis of the IL 1-butyl-3-methylimidazolium dimethyl phosphate, which is well-known to dissolve pulp, the phosphate anion was modified by substituting one oxygen atom for sulfur and selenium, respectively. This alteration reduces the hydrogen bond basicity of the IL and therefore prevents dissolution of cellulose fibers, whereas the less ordered xylan is still dissolved. (1)H NMR spectra of model solutions and Kamlet-Taft parameters were used to quantify the solvent polarity and hydrogen bond acceptor properties of the ILs. These parameters have been correlated to their ability to dissolve xylan and cellulose, which was monitored by (13)C NMR spectroscopy. It was found that the selectivity for xylan dissolution increases to a certain extent with decreasing hydrogen-bond-accepting ability of anions of the ILs.

Accessibility, reactivity and supramolecular structure of E. globulus pulps with reduced xylan content

The correlation of structural assembly on a molecular level with macroscale properties such as accessibility and reactivity was investigated. A series of TCF-bleached E. globulus kraft dissolving pulps was prepared aiming at a specification suitable for viscose application. The removal of xylan to a comparable level was achieved by different pre- and post-treatments. Solid-state CP-MAS 13C NMR was used to determine the degree of order and the lateral fibril dimensions of cellulose fibrils. The results of the NMR measurements were related to the processability of these pulps during viscose manufacture, expressed in terms of filterability of the viscose dope and its amount of undissolved particles. The cellulose crystallinity did not affect the pulp reactivity. It was noticed that the cold caustic extracted (CCE) pulps revealed both large fibril aggregate width as determined from NMR data and low reactivity toward xanthation at the same time. These pulps exhibited significantly higher amounts of alkali-resistant xylan than those prepared by prehydrolysis kraft cooking.

Novel insight into cellulose supramolecular structure through 13C CP-MAS NMR spectroscopy and paramagnetic relaxation enhancement

(13)C CP-MAS NMR and paramagnetic relaxation enhancement provide novel insight into the supramolecular structure of solid cellulose I and II. Separable NMR signals associated with crystalline interiors, solvent accessible and inaccessible surfaces, as well as non-crystalline material are assigned and confirmed. For the first time solvent accessibility is evidenced and monitored through (13)C T1 NMR relaxation enhancement in paramagnetic medium. Established NMR signal assignments for cellulose I have been confirmed. Existing cellulose II resonance attributions have been modified and extended. Novel spectral fitting routines for cellulose II allow for the reproducible quantification of separable signal contributions. Results from NMR line shape analyses are straightforwardly introduced into a model for cellulose II supramolecular structure.

Determination of the xanthate group distribution on viscose by liquid-state 1H NMR spectroscopy

An analytical method for determination of the xanthate group distribution on viscoses based on liquid-state NMR spectroscopy was developed. Sample preparation involves stabilization of the xanthate group by allylation followed by derivatization of the remaining free hydroxyl groups at the glucose unit. The method was applied for studying (1) the γ-value (number of xanthate groups per 100 glucose units) of viscose, (2) the distribution of the xanthate groups on the anhydroglucose unit (AGU), and (3) changes of the xanthate group distribution during ripening. Results of the γ-value determination are well comparable with reference methods. Elucidation of the xanthate group distribution on the AGU gives the percentage at the C-6 position and a cumulative share of the positions C-2 and C-3. During ripening, xanthate groups at C-2 and C-3 degrade first, while xanthates at C-6 decompose at a slower rate.

Toward multipurpose NMR experiments

Standard phase cycled NMR pulse sequences were generalized such that for each individual step of the pulse phase cycle the free induction decay is stored separately without phase correction. This is in contrast to the usual practice, where pulse responses are phase shifted immediately (by applying a ‘receiver phase’) and co‐added as they are stored. The approach used here allows one to extract different types of NMR information, which are usually referred to as different ‘experiments’, from the same raw data set a posteriori by using complex linear combinations. Storing the free induction decays of individual phase cycle steps separately and using specific linear combinations of these data to obtain a particular type of information increase the overall efficiency of a given set of NMR experiments substantially, because all information can be derived from a single multiplexed data set. This ‘super‐experiment’ requires only as much time as the most complex of the derived specific experiments alone.

MQD—Multiplex-Quadrature Detection in Multi-Dimensional NMR

With multiplex-quadrature detection (MQD) the tasks of coherence selection and quadrature separation in N-dimensional heteronuclear NMR experiments are merged. Thus the number of acquisitions required to achieve a desired resolution in the indirect dimensions is significantly reduced. The minimum number of transients per indirect data point, which have to be combined to give pure-phase spectra, is thus decreased by a factor (3/4)N−1. This reduction is achieved without adjustable parameters. We demonstrate the advantage by MQD 3D HNCO and HCCH-TOCSY spectra affording the same resolution and the same per-scan sensitivity as standard phase-cycled ones, but obtained in only 56 % of the usual time and by resolution improvements achieved in the same amount of time.