摘要:

UV absorption spectroscopy was used to determine the chromophore composition of tetrahydrof uran/water-soluble milled wood lignin (MWL) and peroxide-bleached lignin (LHO) using ionization difference UV spectroscopy. MWL and LHO were treated in solution with sodium borohydride to reduce the carbonyl chromophores which were shown to be prone to induce photoyellowing. Furthermore, the reduced solutions were hydrogenated in solution over Pd/C catalyst to eliminate the aromatic-conjugated double bonds that also induce photodiscoloration. The lignins were incorporated into 2-hydroxypropylcellulose (HPC) at a concentration near 0.5% and then irradiated with filtered light (λ > 300 nm) in the form of solid, transparent films. The photochemistry of the different samples was followed by UV/Vis absorption spectroscopy and the kinetics of discoloration and aromatic ring destruction were established. The peroxide-bleached, reduced, and hydrogenated lignin was found to be almost completely stable in the solid carbohydrate matrix under irradiation. Furthermore, the kinetics show the importance of phenolic biphenyl chromophores and probably also of phenolic phenylcoumaran structures in the photoyellowing process.

ISSN:0277-3813    

DOI:10.1080/02773819208545047

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Fluorescence spectra and emission quantum yields of 2-hydroxypropyl-cellulose films, incorporating milled wood lignin that had been treated in solution by HgOg/NaOH and/or NaBfy and/or H2(Pd/C), were measured before and after irradiation by UV light (λ> 300 nm). Bleaching, reduction (NaBH4), and hydrogenation (H2, Pd/C) increase the quantum yield of fluorescence and emission in the blue region (400 nm). The destruction of carbonyl chromophores (α-carbonyl, coniferaldehyde, and quinones), which quench the fluorescence of biphenyl groups, the main structures emitting in this part of the spectrum, appeared to be mainly responsible for this increase. Irradiation restores i) emission in the long wavelength part of the spectra (maximum emission: 500 nm, maximum excitation: 400 nm), and ii) quenching of the fluorescence in the blue part (400 nm) of the spectra, even for hydrogenated films. These results are interpretated in relation to the formation, under irradiation, of coniferaldehyde groups and also the generation of complex structures formed by photooxidation of phenolic biphenyls.

ISSN:0277-3813    

DOI:10.1080/02773819208545048

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The light-induced degradation of lignin was studied using spruce MWL impregnated on handsheets prepared from cotton linters. The sheets were irradiated with simulated sunlight and the lignin was extracted. Solutions of untreated and irradiated lignin were studied by1H and13C NMR spectroscopy. The results showed that α- and β- ether and α-β bond cleavage were the main reactions. The main degradation products were vanillin and vanillic acid. The amount of terminal groups containing aromatic aldehyde and carboxyl groups also incre- ased on irradiation. The existing α- C=O groups were transformed mainly to carboxyl groups. However, the results indicate that new α- C=O groups were generated on irradiation. The methoxyl content decreased and some degradation of aromatic rings took place. However, no formation of quinoid structures was observed. Degradation of phenylcoumaran units was evident, α- Ether cleavage in phenylcoumarans could possibly lead to formation of stilbenes and stilbene derived chromophores.

ISSN:0277-3813    

DOI:10.1080/02773819208545049

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The nature of the solubilized oligosaccharides released from the softwood Plnus radiata by the steam explosion process in the presence or absence of sulphur dioxide are described in this paper. Steam explosion in the absence of sulphur dioxide (215δ, 3 min) resulted in partial solubilization of only the hemicelluloses (14.5 g of neutral sugars/100 g o.d. wood) to their respective oligomers, which ranged in degree of polymerization from 1 to at least 12. The effect of adding 2.5% sulphur dioxide to the substrate was to facilitate the removal and hydrolysis of both hemicellulose and cellulose components (29.5 g of neutral sugars/100 g o.d. wood) by acid catalysis. Steam explosion of the substrate at an elevated temperature of 248° in the presence of 2.5% sulphur dioxide, caused almost complete cellulose and hemicellulose solubilization, and subsequent degradation reflected in poor carbohydrate survival (24.7 g of neutral sugars/100 g o.d. wood). The addition of sulphur dioxide to the process results in enhanced hydrolysis of the solubilized material to mainly mono- and disaccharides. However, the acid reversion products such as isomaltose and gentiobiose are formed, although their effect on total fermentable sugar yield is very small. Under normal steam explosion conditions the acidic aldooligouronic acid hydrolysis products represent only about 6% of total soluble carbohydrate. Interestingly, some demethylation of 4–0-methyl-D-glucuronic acid residues were observed as a consequence of sulphur dioxide catalysis.

ISSN:0277-3813    

DOI:10.1080/02773819208545050

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Mixed hardwood chips were treated with difunctional compounds as catalysts to study the reaction of wood with steam. The Rapid Steam Hydrolysis (RASH) pretreatment process was used for steam treatment. The difunctional compounds studied were maleic anhydride, phthalic anhydride, isophthalic acid, and terephthalic acid at 1.5% concentration based on dry wood weight. RASH pretreatment was performed for one minute at 180°C, 200°, 220°C, 230°C, 240°C, and 260°C. These compounds strongly modified the RASH pretreated material, especially the physical structure. Overall recovery of the pretreated catalyzed and uncatalyzed solids decreased with an increase in RASH temperatures. Catalyst addition did not make a difference on the recovery of pretreated solids. Cellulose degradation increased with temperature for catalyzed systems. Hemicellulose solubilization and degradation were extremely sensitive to the type of catalyst and RASH temperatures. Almost all of the hemicellulose was lost at higher temperatures. Lignin losses did not appear to be affected by the addition of catalyst except at 260°C. Enzymatic rates were improved by addition of the catalysts, especially at the lower temperatures. The maleic anhydride gave the highest enzymatic rates at all temperatures, and phthalic anhydride gave the second highest. The water solubles generally followed the same trends as the enzymatic hydrolysis rates and increased with the addition of catalysts, especially maleic anhydride.

ISSN:0277-3813    

DOI:10.1080/02773819208545051

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A fraction of lignin which was almost mono-disperse was isolated from an acetylated birch periodate lignin, and its chemical structure was investigated by various NMR experiments. It was proved that the fraction was composed mainly of non-phenolic syringyl-β-0-4 structures.

ISSN:0277-3813    

DOI:10.1080/02773819208545052

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The validity of a previously proposed kinetic model for alkaline peroxide bleaching has been tested by changing reagent concentrations during bleaching. The model has been revised to account for experimental observations and now includes a reversible reaction between chromophoric and leucochromophoric groups. Analysis of model first order rate constants indicates that ionic and radical species may both play an important role in alkaline peroxide bleaching.

ISSN:0277-3813    

DOI:10.1080/02773819208545053

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor