Danielle Robert

Structural Changes in Lignin During Kraft Cooking Part 3. On the Structure of Dissolved Lignins

Abstract Two series of pine kraft lignins were prepared by a) normal kraft cooks to different pulp yield levels and precipitation of the lignins from the black liquors by acidification and b) by successive acidification of the black liquor obtained from a flow-through cook. All the lignins were extensively purified, subjected to elemental and methoxyl analysis and subsequently acetylated. Quantitative 13C-NMR analysis was carried out on acetylated samples and the results were combined with the results of phenolic group determination by means of aminolysis and with elemental analysis data. The various acetylated lignins were also subjected to analysis by size exclusion chromatography. All results are discussed with reference to known features of kraft cooking and of kraft lignins.

Substituent Effects on C-13 Chemical Shifts of Aromatic Carbons in Biphenyl Type Lignin Model Compounds

Abstract Simple 5–5 type lignin model compounds have been synthesized from dehydro-divanillin and its ether derivatives by decarbonylation, hydrogenation, and reduction with metal hydride. The chemical shifts for the aromatic carbons of these compounds were assigned, and the substituent effects have been elucidated from 13C NMR spectra of guaiacyl-type monomeric and 5–5 type lignin model compounds. In addition, evaluation of the observed values of substituent chemical shift (SCS) for the aromatic carbons leads to formulation of a generalized SCS additivity rule for the aromatic carbons in 5–5 type substructures. The rule is complementary to the similar rule formulated earlier for the β-0−4 and β-5 type lignin substructures. The rule can be used for the estimation of the chemical shifts of aromatic carbons in lignin model compounds and lignin preparations with reasonable accuracy. On the basis of the observed 13C NMR spectral data for aromatic

Distribution of erythro and threo forms of different types of β‐O‐4 structures in aspen lignin by 13C NMR using the 2D INADEQUATE experiment

Carbon–carbon connectivity spectra of 13C‐enriched aspen lignin recorded using the 2D INADEQUATE experiment revealed cross peaks which can be assigned to four types of arylglycerol β‐aryl ethers (β‐O‐4 structures): erythro forms of arylglycerol β‐syringyl ethers, threo forms of arylglycerol β‐syringyl ethers, erythro forms arylglycerol β‐guaiacyl ethers and threo forms of arylglycerol β‐guaiacyl ethers. The intensities of the cross peaks suggest larger amounts of β‐syringyl ethers than β‐guaiacyl ethers. The erythro isomers dominate among the β‐syringyl ethers. Erythro and threo forms of β‐guaiacyl ethers are present in similar amounts.

Molecular Modeling of Lignin β-O-4 Model Compounds. Comparative Study of the Computed and Experimental Conformational Properties for a Guaiacyl β-O-4 Dimer

Summary The conformational preferences of the threo and erythro diastereomeric forms of a guaiacyl β-O-4 dimer have been investigated by molecular modeling using the CHARMM force field. Many low energy conformations have been identified for each diastereomer, showing that β-O-4 dimers can adopt a large variety of shapes. A consistent structural model has emerged that indicates different conformational behavior for the threo and erythro forms, corresponding to a preferential extended overall shape for the threo form. All the low energy conformers are stabilized by intramolecular H-bonds. In particular, the highly directional H-bond between the α or γ hydroxyl hydrogen and the aromatic methoxy oxygen governs the B aromatic ring orientation. However, it has appeared that the conformational preferences are predominantly governed by local steric interactions rather than by differences in the H-bonding pattern. From the satisfactory agreement between computed data (geometries and the Boltzmann distribution of the low energy conformers) and the experimental data reported in the literature (X-ray crystal structures and 3 J HαHβ NMR coupling constant), the CHARMM force field has been validated for the study of β-O-4 structures. Clearly, the molecular modeling calculations have led to an improved rationalization of the conformational data collected by experimental techniques.

Application of three-dimensional HMQC-HOHAHA NMR spectroscopy to wood lignin, a natural polymer

Abstract The three-dimensional HMQC-HOHAHA experiment was successfully applied to an acetylated 13C-enriched poplar wood lignin preparation. The resolution of this technique proved to be sufficient to give unambiguous assignments for most of the side-chain structures, which overlap heavily in one- and two-dimensional NMR spectra. By this technique it was possible to show, for the first time, that α,β-diaryl ether structures exist in lignin, although in low abundance. These structures have been considered to be of importance in lignin structure, since they may act as reactive cross-linking groups providing the lignin polymer with a branched network structure.

On the Behaviour of Spruce Thermomechanical Pulp Lignin during Hydrogen Peroxide Bleaching

Lignins were isolated from spruce TMP and hydrogen peroxide bleached TMP (BTMP) by means of enzymatic hydrolysis with cellulases in yields of 67 and 78%, respectively. In addition, the TMP lignin, TMP EL, was subjected to a treatment with bleaching agent to obtain a bleached lignin, TMP ELB. The three lignin samples were all characterized by thioacidolysis and C-NMR spectroscopy. Comparison of these lignin samples provided evidence about the behaviour of lignin towards alkaline hydrogen peroxide bleaching. It has been demonstrated that the bleaching does not affect the overall structure of lignin. The only one major change of the lignin moiety is elimination of coniferaldehyde end structures, which would be an important contribution to the brightening of pulp.

Reactions of Lignin in Chlorine Dioxide Bleaching of Kraft Pulps

The reactions between chlorine dioxide and the residual lignin in oxygen-bleached softwood kraft pulps have been studied. In a first series, isolated lignin samples have been subjected to chlorine dioxide oxidation at different pH values and subsequently analysed by oxidative degradation and elemental analysis. Different analytical techniques have also been employed to follow the gradual chemical changes in lignins isolated from kraft pulps after each of the bleaching stages in the OD(EOP)DD sequence.The results demonstrate that, in order to minimize chlorination of the lignin, the first chlorine dioxide stage should be carried out at a pH around or above three. At this pH level, a high degree of lignin oxidation is also achieved. A certain (mono)-chlorination of the lignin in the first D stage cannot be avoided, but this chlorine is to a large extent removed in the later bleaching stages. The efficient and non-selective oxidation of the various phenolic lignin end groups by chlorine dioxide is clearly illustrated by the analytical data. Moreover,13C NMR reveals that reduced lignin structures formed during the kraft cook survive the oxidative bleaching stages to a large extent.

Synthesis of Lignin-Related Cinnamaldehydes

Cinnamaldehyde units are present in lignins (Adler 1977) and certain types of cinnamaldehydes are therefore of interest as lignin model compounds. Lignin-related cinnamaldehydes [such as, coniferaldehyde (1), pcoumaraldehyde (2) and sinapaldehyde (3)] are well known lignin hydrolysis products and are also frequently found in plant extractives. The bioactivity of 1–3 has been examined in several recent papers (see e.g., Leem et al.1999; Barber et al. 2000). The synthesis of monomeric (Iliefski et al. 1998) and dimeric (Lundquist and Hedlund 1971; Li et al. 1998) lignin-related cinnamaldehydes by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) oxidation of 1-aryl-1-propenes and 3-aryl-1-propenes has been described. Synthesis of phenolic cinnamaldehydes such as 1–3 by this method involves the protection and deprotection of a phenolic group. We have synthesized 1–3 by DDQ oxidation and in this context we have studied the applicability of different types of protective groups. Different methods for the introduction of the protective groups were also examined. We think that the synthetic methods developed are generally applicable to the synthesis of phenolic lignin-related cinnamaldehydes. Lignin-related cinnamaldehydes and their properties are also of interest in connection with studies of lignins of cinnamyl alcohol dehydrogenase (CAD) deficient plants (Ralph et al. 2001a;Li et al. 2001). In this context model compounds of the α-aryloxycinnamaldehyde type are of interest. The synthesis and properties of two compounds of this type (18 and 21) are described in this paper.