摘要:

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 from13C 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 observed13C NMR spectral data for aromatic

ISSN:0277-3813    

DOI:10.1080/02773818908050309

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Residual lignins were isolated from cooking residues obtained from 20 1 batch cookings of spruce chips by either ball-milling or enzymatic treatment. The lignins were characterized in terms of yield, elemental composition, polysaccharide moieties and functional groups and compared with the corresponding dissolved lignins.

ISSN:0277-3813    

DOI:10.1080/02773818908050310

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Computer simulations of kinetic phenomena have been carried out for a general soda-additive delignification mechanism consisting of an equilibrium process, followed by an irreversible step in which the additive participates. Simulations have been performed under conditions where the concentration of additive is either at steady state, or declines with time. The results clearly show that caution should be exercised in interpretation of experimental data when attempting to use kinetic results to elucidate details of reaction mechanisms. In particular, it is shown that the origin of exprimentally observed dependencies on additive concentrations such as a “square-root” relationship can be explained by assuming different ratios of rate constants for formation and subsequent decomposition of a quinone methide intermediate.

ISSN:0277-3813    

DOI:10.1080/02773818908050311

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Lignin and lignin-related compounds were oxidized with potassium nitrosodisulfonate, hydrogen peroxide, and peracetic acid, with the aim of producingo- andp-benzoquinones. Depending on the substrate and oxidant, yields ofp-benzoquinones from lignin model compounds were as high as 80–95%. The best yield ofp-benzoquinones from lignin (16%) was with the use of potassium nitrosodisulfonate and an aspen hardwood lignin; corresponding yields obtained with hydrogen peroxide and peracetic acid were 0–2%. Electrochemical oxidation was examined briefly and found to be inferior to the chemical oxidants with regard to yields of benzoquinones from lignin models (max. 40%) and from lignin (1%). The nitrobenzene oxidation of several lignin samples revealed that the highest syringaldehyde and combined aldehyde yields (for subsequent oxidation to quinones) were obtained from an organosolv lignin obtained through the acid-catalyzed pulping of aspen with ethanol.

ISSN:0277-3813    

DOI:10.1080/02773818908050312

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The Diels-Alder reactions of methoxy-p-benzoquinone and 2,6-dimethoxy-p-benzoquinone, which may be obtained from lignin and lignin model compounds, and ofo-quinones (4-methyl, 4-acetyl, and 4-(l-hydroxyethyl)-o-benzoquinone) were investigated using 2,3-dimethylbutadiene, isoprene and styrene as dienes. Dimethylbutadiene and isoprene were found to be effective reactants, resulting in the generation of several Diels-Alder addition products. Loss of the methoxyl group and the side chain were observed under certain conditions. Reactions of thep-benzoquinones went more smoothly than the reactions of theo-quinones, and product yields were generally higher. Higher yields of anthraquinone products were obtained from 2,6-dimethoxy-p-benzoquinone than from methoxy-p-benzoquinone.

ISSN:0277-3813    

DOI:10.1080/02773818908050313

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Quinones, which may be generated from lignin and lignin-related materials, and their Diels-Alder derivatives were evaluated as delignification promoters. Compared to anthraquinone at a 0.1% addition level, benzoquinones and oxidized lignin showed little or no effect. However, some naphthoquinones were found effective at higher addition levels, and a mixture of dimethyl-anthraquinone isomers was found to be superior to anthraquinone. The substituted anthraquinones, phenanthrenequinones, and fully aromatized naphthoquinones appear to function in a reduction/oxidation cycle such as that proposed for anthraquinone.

ISSN:0277-3813    

DOI:10.1080/02773818908050314

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The acid-soluble lignin determination used in analyses of lignin by the sulfuric acid method was evaluated. The conventional UV spectrophotometric method using the absorption at 205 nm is appropriate for hardwoods, whereas for softwoods and bamboo the absorption at 205 nm should be corrected, since the absorption maximum is shifted to a shorter wavelength in sulfuric acid solution. Therefore, in the latter cases, the actual absorption maximum should be used.

ISSN:0277-3813    

DOI:10.1080/02773818908050315

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Adducts were prepared by reacting wood cell wall polysaccharide, lignin and model compounds of these polymers with chromium(vi) ions. Magnetic susceptibilities of these adducts were measured using a Gouy balance. It was shown that under the reaction conditions used, (polysaccharide, lignin, etc. in molar excess), the adducts were paramagnetic. This implies that the chromium in the adducts is in the tervalent state.

ISSN:0277-3813    

DOI:10.1080/02773818908050316

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

This is a scanned image of the original Editorial Board page(s) for this issue.

ISSN:0277-3813    

DOI:10.1080/02773818908050308

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor