摘要:

A key step in the delignification of wood is the breakage of the β-aryl ether bonds of lignin. Two mechanisms are discussed for how anthrahydroquinone (AHQ) brings about this particular fragmentation. The “adduct” mechanism involves bond formation between lignin quinonemethide (QM) intermediates and AHQ, followed by fragmentation. The other mechanism (“SET” mechanism) involves a single electron transfer between AHQ and a lignin QM followed by fragmentation. The literature concerning adducts and SET reactions is reviewed and analyzed. The SET mechanism must be considered as a viable alternative to one based entirely on adduct formation.

ISSN:0277-3813    

DOI:10.1080/02773818508085178

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Electron transfer reactions have been observed during the electrolyses of solutions containing anthraquinone and 8-aryl ether lignin model quinonemethides. In dry acetonitrile at a reduction potential of -0.9V (vs. Ag/AgCl) electrons are transferred from the electrode to anthraquinone (AQ) to form stable anthrahydroquinone radical anions (AHQ−). The lignin model quinonemethides are not reduced directly at the electrode at this potential but are reduced by AHQ−to give quinonemethide radical anions (QM−) and AQ. The QM−species rapidly fragment at their β-aryl ether bond to give phenolate ions and radicals; the latter further reduces to another phenolate ion. For example, the β-methyl lignin model QM 1 gives guaiacol and isoeugenol upon electrolysis at -0.9V in the presence of AQ. In wet acetonitrile, reduction of AQ at -0.9V leads to both anthrahydroquinone radical anion and dianion; the dianion is formed by direct electrolysis of the radical anion and by disproportlonation of the radical anion. Under all conditions and substrates examined, electron transfer reactions proceeded in preference to bond formation reactions which would generate “adducts.” The implication of these results Is that it should be possible to delignlfy wood by electron transfer reactions and that anthraquinone probably functions this way.

ISSN:0277-3813    

DOI:10.1080/02773818508085179

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

The condensation of coniferyl alcohol with dissolved lignins of low molecular weight (LMWL) has been investigated. In aqueous sodium hydroxide (soda), sodium hydroxide - sodium sulfide (kraft) or sodium hydroxide - anthraquinone (soda-AQ) solutions, lignin preparations were heated and molecular weight distributions of the resultant condensation products were estimated. When coniferyl alcohol dissolved in soda and kraft liquors was heated, self-condensation was observed, mainly to form oligomers with only a small proportion of the reaction products characterized by molecular weights greater than about 1000. When mixtures of LMWL and coniferyl alcohol were heated in sodium hydroxide solutions, condensation reactions occurred and progressed further with increased temperature and concentration of coniferyl alcohol. In contrast to results with sodium hydroxide solutions, heating in kraft liquors gave rise to a higher proportion of the oligomer fraction and a smaller proportion of higher molecular weight entities. These results seem to indicate that the presence of sulfide reduces the extent of condensation reactions in alkaline pulping. With soda-AQ solutions, condensation effects were found to be intermediate between those observed with soda and kraft liquors.

ISSN:0277-3813    

DOI:10.1080/02773818508085180

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

In order to elucidate the main reactions of ozone with residual lignins in softwood kraft pulps, model compounds representing conjugated structures (styrenes and stilbenes) and structures of the biaryl, diary] ether and diarylmethane types have been treated with this oxidant.

ISSN:0277-3813    

DOI:10.1080/02773818508085181

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

A pyrolytic study of a variety of lignins prepared from hardwood and softwood species, and of selected commercial lignins, was performed using isothermal and dynamic thermogravimetric analysis and pyrolysis-gas chromatography. The lignins were evaluated according to weight loss, rate of weight loss, and products formed during the pyrolysis in an inert atmosphere. These data were then compared to nitrobenzene oxidation data. Infrared spectroscopy was used to determine structural changes of lignin during initial weight loss in the temperature range of 250° to 320°C. Structural changes appear to occur without any appreciable weight loss. Lignin's thermal behavior during pyrolysis depends upon the degree of condensation present in the initial lignin. Thermogravimetric analysis is shown to be a simple and accurate method to determine the extent of lignin condensation. The yield of monomeric phenols is also dependent on the degree of lignin condensation.

ISSN:0277-3813    

DOI:10.1080/02773818508085182

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

The explosion treatment has been used with the objective of increasing the accessibility of wood. After the explosion treatment wood meal was esterified. Acyl groups introduced were acetyl, caproyl and lauroyl. Caproylation was carried out by three different methods - PCA catalyst method, TFAA catalyst method and DMF/PY/acid chloride method. The PCA catalyst method was not used for lauroylation since it is not an effective procedure for introducing higher acyl groups. The DMF/PY/acid chloride method could not be used successfully for acetylation. Thermal softening and apparent melting behaviour of the exploded-esterified wood meal was studied by following the collapse of a column of powder under a constant load in a glass capillary tube 3 mm in diameter at a programmed rate of heating.

ISSN:0277-3813    

DOI:10.1080/02773818508085183

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Reaction products of phloroglucinol or catechin with furfuryl alcohol and furfuraldehyde were studied. In reactions of furfuryl alcohol with phloroglucinol, only 2-furyl-1′,3′,5′-trihydroxyphenyl) methane was obtained as product, and 53% of the phloroglucinol was recovered. Reactions of furfuryl alcohol with catechin gave 2-furyl-(8-catechinyl) methane and 2-furyl-(6-catechinyl) methane in 4.0% and 1.5% yields, respectively, while 62% of the catechin was unreacted. Polymeric furans with few catechin moieties made up the oligomeric products (38% of catechin). Reaction of phloroglucinol with furfuraldehyde gave 2-furyl-di(1′,3′,5′-trihydroxyphenyl) methane, an unstable product that readily polymerized during isolation. The solid state13C-NMR spectrum of the higher polymers suggested one phloroglucinol moiety per furan unit, but lower oligomers contained more furan-furan condensation products. Reactions of catechin with furfuraldehyde gave 2-furyl-di(8-cate-chinyl) methane and the two diastereomers of 2-furyl-(6-catechinyl)-(8-catechinyl) methane in low yield, with 65% of the catechin unreacted.

ISSN:0277-3813    

DOI:10.1080/02773818508085184

出版商:Taylor & Francis Group    

年代:1985

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:10.1080/02773818508085177

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor