摘要:

Bleached softwood mechanical pulp was treated with sodium borohydride to reduce carbonyl groups and dimethyl sulphate to block phenolic hydroxyl groups. Upon irradiation with near ultra-violet light, new phenolic hydroxyl groups and aromatic carbonyl groups formed. Existing mechanisms for photoyellowing require that either aromatic ketones or phenolic hydroxyls be initially present in the pulp, and cannot explain these observations. We interpret our results in terms of light-induced breakdown of etherified arylglycerol-β-arylether structures in lignin. Degradation occurs via the corresponding ketyl radicals, which cleave rapidly at the β-O-4 bond to give a phenoxy radical and an acetophenone enol. The enol tautomerizes to a ketone, while the phenoxy radical is oxidized to coloured groups. We believe that the ketyl route is the main lignin degradative pathway, and estimate that up to 70% of the colour formed during light-induced yellowing is attributable to this reaction.

ISSN:0277-3813    

DOI:10.1080/02773819308020520

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Integral ESR and time-resolved CIDEP techniques applied to the study of the light-induced yellowing of bleached and unbleached TMP samples provide some further insights into the nature and the mechanisms of the processes. The identity of the phenoxy and ketyl free radical intermediates involved in mese processes is established and the primary triplet photochemical mechanisms of their formation are characterized. The radical scavenging mechanisms responsible for the inhibition of light-induced yellowing in the TMP system by ascorbic acid and triphenylmethyl thiol are formulated.

ISSN:0277-3813    

DOI:10.1080/02773819308020521

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The effect of chemical pretreatment on the chemical characteristics of steam explosion pulps was studied by ESCA, FTIR and other chemical methods. 18 different chemical solutions, which were the combination of following chemicals Na2SO3, NaOH, NaHCO3, Na2CO3, MgCO3, MgCl2, ZnCl2, Na2S or H2O2, were used as pretreatment agent. The chemical characteristics of pulps with different pretreatment including hydrophilic group content, surface composition of the fibers, bulk chemical composition as well as physical and optical properties were compared. Compared to Na2SO3alone or to other chemical pretreatment systems, a presence of NaOH, NaHCO3or MgCO3led to a higher hydrophilic group content, more carbohydrates exposed on the fiber surface as well as to a higher degree of lignin sulfonation, which may explain their high physical strength. A lignin analysis showed little difference in their bulk lignin contents. The presence of NaOH in impregnation solution led to xylan partially dissolving during the steam explosion cooking process. Furthermore, controlling the impregnation solution's initial pH in the range 9–10 normally led to high pulp yield.

ISSN:0277-3813    

DOI:10.1080/02773819308020522

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Cleavage of the β-aryl ether bonds of the lignin model compounds guaiacylglycol β-guaiacyl ether (1) and guaiacylglycerol β-guaiacyl ether (11) to form guaiacol was found to be catalyzed by the water soluble porphyrin complex trisodium meso-tetra-4-sulfonatophenvlporphinecobalt(III), (Co(TSPP)). The resulting product distributions were remarkably similar to those obtained for the anthraquinone (AQ) catalyzed reactions under the same conditions. Studies of the reactions of CofTSPP) with proposed intermediate compounds indicate that the major route to guaiacol formation involves reaction of the reduced catalyst, CoII(TSPP), with the quinone methides 9 and 13 which are derived from the phenolic model compounds 1 and 11 respectively. It is proposed that the mechanisms of these reactions involve single electron transfer to the quinone methide as the first step. The implications of this proposal for the AQ catalyzed reactions of 1 are discussed.

ISSN:0277-3813    

DOI:10.1080/02773819308020523

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The water soluble phthalocyanine complex trisodium tetra-4-sulfonatophthalocyanineiron(III) (Fe(TSPc)) was found to be an effective catalyst for the cleavage of the β-ether bonds in the phenolic lignin model compounds guaiacylglycol β-guaiacyl ether (1) and guaiacylglycerol β-guaiacyl ether (11). The products of these reactions were very different from those formed in the corresponding reactions catalyzed by anthraquinone (AQ) or Co(SPP).1–4In particular, they gave large quantities of oxidized products, even though the reactions were performed in the absence of oxygen or other added oxidant. Mechanisms have been proposed for the oxidation reactions involving 1 and 11. In both cases the first step involves one electron oxidation of the lignin model compound by the catalyst. The radical derived from 1 then undergoes further one electron oxidation and deprotonation to give 4′-hydroxy-3′-methoxy-l-(2″-methoxyphenoxy)acetophenone (8) whereas that derived from 11 undergoes Cα-Cβ bond cleavage to give vanillin (4). Reactions of the reduced form of the catalyst with 8 and the quinone methides produced from the phenolic models are important routes for guaiacol formation and regeneration of the oxidized form of the catalyst. The feasibility of these proposed reaction pathways was investigated by studying the reactions of the intermediate compounds with the catalyst.

ISSN:0277-3813    

DOI:10.1080/02773819308020524

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Trisodium meso-tetra-4-sulfonatophenylporphinerhodium(III) (Rh(SPP)) was found to be a very effective catalyst for cleaving the β-aryl ether bonds of the lignin model compounds guaiacylglycol β-guaiacyl ether (1) and guaiacylglycerol β-guaiacyl ether (11). In both cases very large amounts of guaiacol (2) were formed regardless of whether the reducing sugar glucose was present or not. With the substituted phenylethane model 1 the other major product was acetovanillone (5); with the substituted phenylpropane model 11 considerable amounts of vanillin (4) were formed. The products obtained with the rhodium catalyst were very different to those formed in the Co(SPP) or anthraquinone (AQ) catalyzed reactions. They were, however, similar to those obtained with Fe(TSPc), although the rhodium catalyst was much more active and formed guaiacol at a much faster rate. In addition to these reactions, the rhodium complex also catalyzes the partial demethylation of the 3, 4-dimethoxyphenyl model compound 10. Mechanisms are proposed for these reactions in which the key steps involve single electron transfer between the catalyst and the substrate.

ISSN:0277-3813    

DOI:10.1080/02773819308020525

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We investigated the degradation of sugars during acid hydrolysis and the effects of other aspects of sample preparation on measurement of wood sugars by anion exchange HPLC using pulsed amperometric detection. Loss of sugars in standards ranged from 6.4% for arabinose to 15.7% for mannose. Measurements of wood samples to which standard was added before hydrolysis were very close to the sum of wood samples and standards hydrolyzed and analyzed separately except in the case of galactose, which was in significantly lower amounts (4.7% lower) in the combined samples. There was greater variance among injections of individual replicates than among replicates. Other, minor procedural details were investigated.

ISSN:0277-3813    

DOI:10.1080/02773819308020526

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Several factors which could affect the accuracy of measurements of sulphur species by ion chromatography have been identified. Analyses of sulphide, sulphite, sulphate and thiosulphate were conducted on samples of kraft liquors, spent sulphite liquor and newsprint white water. Erroneously high results were observed in the determination of sulphide in kraft white and green liquors, presumably due to shifts in equilibrium between elemental sulphur and polysulphide ions during sample dilution. Another problem was encountered during the analysis of sulphite in kraft liquors. Sulphite concentrations in diluted liquors rose rapidly with increase in time between dilution and injection into the ion chromatograph. Evidence presented in this paper suggests that, in diluted kraft liquors containing polysulphide and/or elemental sulphur, the sulphite concentration increases rapidly, possibly due to hydrolysis of elemental sulphur. Determination of thiosulphate was also subject to positive errors, due to oxidation of sulphide to thiosulphate. The effect of storage conditions on the stability of sulphur species was also determined. Room- temperature storage was the most deleterious for white water samples containing low levels of sulphur ions. Long-term stability in all samples was much improved by refrigeration or freezing. Kraft liquors were very stable even when stored at room temperature. Various recommendations are made to maintain sample integrity and minimize analysis errors.

ISSN:0277-3813    

DOI:10.1080/02773819308020527

出版商:Taylor & Francis Group    

年代:1993

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:10.1080/02773819308020519

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor