摘要:

Photoreactions of two dilignols, a trilignol and two tetralignols, all of which contain a phenacyl aryl ether type structure, were investigated. All lignols in ethanol yielded products showing the homolytic cleavage of a phenacyl aryl ether bond in this structure. As to tetralignols photodiscoloration was hardly observed in the absence of air, and the preexistence of a free phenolic hydroxyl groups showed no influence on discoloration even in the presence of air. Photoreaction on a filter paper revealed that main reactions are the recombination of radicals once formed by irradiation and the reaction of radicals with oxygen.

ISSN:0277-3813    

DOI:10.1080/02773819108050275

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A range of guaiacyl and syringyl lignin model phenols was treated with oxygen in 1M potassium hydroxide solution at 70°C. The reactions were monitored by high performance liquid chromatography and gas chromatography-mass spectrometry. The reactions of the phenols, which followed pseudo-first-order kinetics, were faster for syringyl than for guaiacyl phenols. For the various 4-substituted syringols the reactivities were in decreasing order CH2-syringyl > CHOH‒CH3π C3H7n> CH2OH > COOH > CHO > CO‒CH3. Reaction of 1-guaiacylpropane in 1M potassium hydroxide with oxygen gave products of oxidative scission of the aromatic ring and no dehydrodimer, whereas at pH 11.5 some dehydrodimer was among the reaction products. Vanillyl alcohol and syringyl alcohol yielded vanillin and syringaldehyde, respectively, as minor oxidation products. However, the reaction sites for the series of phenols were generally the aromatic rings rather than the side-chains. Oxidation of alkaline solutions of the phenols with oxygen at 1.0 MPa pressure and 110 and 150°C gave similar mixtures of acids and hydroxyacids.

ISSN:0277-3813    

DOI:10.1080/02773819108050276

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Methyl β-D-ribopyranoside (1) and methyl β-D-xylopyranoside (2) were degraded by oxygen (0.682 MPa partial pressure) in 1.25Msodium hydroxide at 120°C. The degradations of1and2were similar to the previously reported degradations of 1,5-anhydroribitol (3) and 1,5-anhydroxylitol (4), respectively.4Both hydrogen peroxide and stable organic peroxides were detected in the reactions of1and2. The riboside1degraded faster than the xyloside2. This difference in reactivity is proposed to be a function of the relative acidity of the glycosides. Ionization of hydroxyl groups is postulated to be favored in1, thus facilitating the initiation of the free radical degradation. The degradations of both1and2exhibited complex kinetics indicating autoinhibited reactions. In spite of the differences in reactivity, glycosidic bond cleavage occurred in approximately 60% of the degradations for both1and2. C-1 radicals, resulting from abstraction of the anomeric hydrogen atom, are proposed to cause the observed autoinhibitionviatermination reactions with α-hydroxyhydroperoxyl radicals. However, decomposition of the glycosidesviaC-1 radicals is not believed to constitute a major degradation pathway since the reactivities of1and2were essentially the same as the analogous 1,5-anhydroalditols, i.e.3and4, respectively. The major acidic degradation products of1and2were identical, but were formed in different relative ratios. The major acidic products were methoxyacetic acid, lactic acid, glycolic acid, glyceric acid, a methyl 3-C-carboxyfuranoside, and two isomeric 2-C-carboxyfuranosides.

ISSN:0277-3813    

DOI:10.1080/02773819108050277

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The addition of chromium nitrate to a two-stage hydrogen peroxide bleach ofEucalyptSGW can enhance the brightness of the pulp. It is proposed that radicals produced in the catalytic decomposition of hydrogen peroxide by chromium under acidic conditions participate in reactions which render the chromophores more susceptible to bleaching in the alkaline stage. This two-stage process, under optimised conditions, consumes no more peroxide than a traditional single stage alkaline bleach and allows the pulp to be bleached to a given brightness in a shorter time. Enhanced bleaching response was also observed for a two-stage acid/alkali peroxide bleaching sequence without the addition of chromium, and we have attributed this effect to catalytic effects of transition metal ions which occur naturally in the pulp.

ISSN:0277-3813    

DOI:10.1080/02773819108050278

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Several kinetic models for alkaline peroxide bleaching of Eucalypt mechanical pulp have been examined. A previously reported kinetic model, developed to describe peroxide bleaching of softwoods, was found to be inadequate. The models proposed here assume two distinct categories of chromophores which are eliminated through first order processes, rather than the high orders for reaction processes suggested in previous kinetic analyses. Our model enables prediction of a maximum in the rate of bleaching in the range pH 11–12, whereas previous models imply that the rate of chromophore elimination increases continuously with hydroxide ion concentration.

ISSN:0277-3813    

DOI:10.1080/02773819108050279

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Chemical structures of underivatized and acetylated birch Milled Wood Lignins were investigated in solution by various two-dimensional NMR experiments such as COSY, HOHAHA, HMQC, and HMBC.

ISSN:0277-3813    

DOI:10.1080/02773819108050280

出版商:Taylor & Francis Group    

年代:1991

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:10.1080/02773819108050274

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor