-Loung Chen

A novel method for conversion of benzyl alcohols to benzaldehydes by laccase-catalysed oxidation

Abstract A new catalytic method for the oxidation of substituted benzyl alcohols to the corresponding benzaldehydes by molecular oxygen using the enzyme/cofactor system laccase/2,2′-azino-bis(3-ethyl-benzothiazoline-6-sulfonic acid) is presented. The reaction proceeds under very mild conditions giving the product in quantitative yields. The enzyme requires at least one free ortho -position in the substrate molecule for the reaction to proceed.

Carboxylic Acids Produced Through Oxidative Cleavage of Aromatic Rings During Degradation of Lignin in Spruce Wood by Phanerochaete Chrysosporium

Abstract Pre-extracted spruce wood chips were decayed by Phanerochaete chrysoaporium to a 20% loss in lignin (8% loss in total weight), and extracted successively with petroleum ether, chloroform, acetone, methanol, and aqueous dioxane. The low molecular weight fraction of the methanol extract was analyzed by gas chromatography/ high resolution mass spectrometry after acetylation and nethylation. Examination of the spectra resulted in structural assignments for 28 compounds, 10 of which were aromatic acids identified and reported previously (Holzforschung 36,3 (1982)). At least 13 of the remaining compounds were formed via aromatic ring cleavage. In addition to ring cleavages, the new structures revealed oxidation of α- and γ-hydroxyl groups, oxidative cleavage of Cα−Cβ and Cβ–Cγ bonds, and 3-O-demethylation. It is postulated that oxidative cleavage of aromatic rings in the lignin units with an ether linkage at C-4 of the guaiacyl group involves 3-O-demethylation, hydroxylation at C-2 and subsequent o-cle...

Oxidation of isoeugenol and coniferyl alcohol catalyzed by laccases isolated from Rhus vernicifera Stokes and Pycnoporus coccineus

Abstract Laccases isolated from Rhus vernicifera Stokes (tree) and Pycnoporus coccineus (fungus) catalyzed the oxidation of isoeugenol ( 1 ) and coniferyl alcohol ( 5 ) in acetone–water (1:1, v/v). These oxidations follow a first order rate law. In general, the rates of Pycnoporus laccase-catalyzed oxidation of 1 and 5 are approximately three and seven times faster than the corresponding rates of Rhus laccase-catalyzed oxidation, respectively. Thus, synthesis for 2-(4-hydroxyphenyl)coumaran type compounds, such as dehydrodiconiferyl alcohol, can be accomplished by Rhus laccase-catalyzed dehydrogenative polymerization of the corresponding 1-(4-hydroxyphenyl)–1-propene derivatives. The reaction proceeds under very mild reaction conditions. The resulting reaction mixtures are chromatographed on a silica gel column to isolate the products in approximately 30–40% yield.

13C NMR Spectroscopic Study of Spruce Lignin Degraded Phanerochaete Chrysosporium

Pre-extracted spruce wood chips were degraded by Phanerochaete chrysosporium to a 20% loss in lignin (8% total weight loss) and extracted successively with petroIeum ether, Chloroform, acetone, mcthanol, and 96% aqueous dioxane. The extracts of the latter three solvents consisted mostly of polymeric and low molecular weight substances derived from lignin. The methanol extract was divided into high and low molecular weight acidic (M-HMWA and M-LMWA), and high and low molecular weight phcnolic (M-HMWP and M-LMWP) fractions. The 96% aqueous dioxane extract was similarly divided, but contained no phenolic fractions. Fractions M-HMWA, D-HMWA (high molecular weight acidic fraction of the dioxane extract), M-HMWP, and milled wood lignin from the nondegraded spruce wood were characterized by C NMR. M-HMWA and D-HMWA were seen to be essentially identical; gel permeation chromatography, however, showed the latter to be of higher molecular weight. On the basis of published C NMR data and studies of modcl compounds (s. Part II), the C NMR Signals in the biodegraded lignins were assigned. It was concluded that fractions M-HMWA and D-HMWA contained 4-0-Alkyl vanillic acid-, 4-0-Alkyl vanillyl alcohol-, aroxyethanol-, aroxyacetic acid-, alkoxyacetic acid-, -carbonyl-, and saturated aliphatic moieties. The Structures point to Ca~Cß side chain cleavages, to Co-oxidations, to aromatic ring cleavage, and possibly to reductive reactions in the polymer during the fungal attack.

Characterization of lignin by 1H and 13C NMR spectroscopy

Publisher Summary This chapter discusses the characterization of lignin by 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy. The latter includes techniques for obtaining routine, quantitative, and distortionless enhancement by polarization transfer (DEPT) 13 C NMR spectra of lignins. Among the various physical and chemical methods for characterization of lignins, 1 H NMR and 13 C NMR spectroscopy has been shown to be among the most reliable and comprehensive techniques. The characterization of lignin by 13 C NMR spectroscopy, in particular, furnishes rather comprehensive data about the nature of all carbons in lignin in terms of chemical structure. By contrast, the other physical and chemical analytical methods only provide incomplete information on the chemical structure of lignin. However, several difficulties are still encountered in the interpretation of the 1 H NMR and 13 C NMR spectra of lignins—for example, the assignment of signals, because of the intensive overlap of signals for 1 H and 13 C nuclei in lignin present in similar, but nonidentical chemical environments. Some of these difficulties can be circumvented by the application of more sophisticated 13 C NMR pulse sequences such as the attached proton test experiment and the DEPT sequence.

Structural Analysis of Residual and Technical Lignins by 1H-13C Correlation 2D NMR-Spectroscopy

Summary Structural analysis was conducted on residual lignin from pine Kraft AQ pulp, Eucalyptus Kraft lignin from Eucalyptus globulus and Repap Organosolv lignin by 2D 13C-1H correlation NMR spectroscopic techniques such as HMQC sequence. These lignins contain a rather wide variety of saturated aliphatic groups. The HMQC NMR spectra of the lignins do not verify the presence of diarylmethane moieties in any lignin investigated. The type and amount of other condensed structures depend on the nature of lignin preparation. All the lignins investigated still contained β-O-4′, pino- and syringayresinol (β-β′) and phenylcoumarane (β-5′) structures. Stilbene structures were also identified. Vinyl ether structures were present only in Eucalyptus Kraft lignin. All the lignins contain α-carbonyl groups conjugated to aromatic moieties as terminal side chains rather than involving β-O-4′ structures. No coniferyl alcohol and coniferyl aldehyde type structures are detected in the lignins after pulping. The spectra of kraft lignins show some new signals, the origin of which is discussed.

Biobleaching of pulp with dioxygen in laccase-mediator system: effect of variables on the reaction kinetics

Comparative studies were carried out on the kinetics and mechanism of pulp biobleaching with laccase-mediator system (LMS) with two different mediators, 1-hydroxybenzotriazole (HOBT) and N-hydroxyacetanilide (NHAA). The optimal NHAA and laccase charge was found to be 0.1 mmol and 10 U per gram of pulp with pulp consistency of 10%, at the reaction temperature of 40 ◦ C for 8 h under atmospheric pressure, respectively. The kinetic studies on Kappa number reduction and dioxygen uptake suggest that a very fast rate of delignification with NHAA at the beginning of the process is the result of fast formation of the oxidized mediator species. However, a very slow delignification rate after the initial phase (0.5–1 h) could be caused by low stability of the mediator species. After the reaction time of 2 h, the degree of delignification is higher when HOBT is used as mediator. In contrast to the delignification with NHAA, the formation of the oxidized mediator species is the rate-determining step of the pulp biobleaching with dioxygen in the LMS using HOBT as mediator. Increase in temperature increases the rate of chemical reactions, but decreases the laccase stability. The optimal temperature for pulp biobleaching with HOBT and laccase from Coriolus versicoloris 40 ◦ C. Increasing oxygen pressure improves the efficiency of delignification due to better penetration of the reagents, but does not affect the rate of chemical reactions. The reaction mechanism is discussed based on the kinetic data.

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

Biobleaching of pulp with dioxygen in the laccase-mediator system — reaction mechanisms for degradation of residual lignin

Abstract Pine Kraft-AQ pulp was biobleached with pressurized dioxygen at 40°C in laccase-mediator system (LMS), i.e. in acetate buffer (pH 4.5) containing Coriolus-laccase and 1-hydroxy-benzotriazole (HOBT), the latter being as a mediator. The LMS-treatment was followed by alkaline extraction (E) under standard conditions. The structures of the residual lignins before and after the biobleaching did not differ appreciably. This indicates that only a part of the residual lignin in the pulp undergoes oxidative degradation in the LMS treatment. In contrast, the treatment resulted in strong changes in the structure of the lignin isolated from E-effluents. The 2D HMQC (1H13C correlation) spectra showed the disappearance of β-O-4′, β-β′ and β-5′ bonds in the structure of the alkaline soluble lignin (ASL) from E-effluents, which are present in the 2D spectrum of the original residual lignin (RKL). In addition, the spectra exhibited new signals that are assigned to ArCOOH in biphenyl (5-5′) moieties. This implies that oxidative cleavage of side chains plays an important role in the delignification of pulp. The NMR studies also indicated that intensive degradation of aromatic ring has occurred in the biobleaching. However, premethylation of neither benzyl alcohol nor phenolic hydroxyl groups of the residual lignin in pulp before the biobleaching affected the rate of delignification. The latter indicates that phenolic moieties participate not only in oxidative degradation but also dehydrogenative polymerization reactions in the biobleaching. This is consistent with an appreciable increase in the proportion of fractions with higher molecular mass in lignin isolated from E-effluents.

A Mn(IV)-Me4DTNE complex catalyzed oxidation of lignin model compounds with hydrogen peroxide

Abstract 1-(3,4-dimethoxyphenyl)ethanol, 1-(3,4-dimethoxyphenyl)-1-propene (mixture of E - and Z -isomers) and E -1,2-diphenylethene were chosen as model compounds to investigate the reactivity of lignin toward hydrogen peroxide catalyzed by [L′Mn(IV)(μ-O) 3 Mn(IV)](ClO 4 ) 2 where L′ is 1,2-bis-(4,7-dimethyl-1,4,7-triazacyclonon-1-yl)ethane. All the model compounds investigated were readily oxidized to a significant extent by hydrogen peroxide when catalyzed by this complex. The reaction products were identified by GCMS, and the reaction kinetics was studied. Based on these results, the mechanisms of the reactions have been elucidated. The catalyst preferentially epoxidates C–C double bonds conjugated with aromatic moieties. The activation energy of the reactions was determined.