摘要:

It has been proposed that the initial regime of pulp ozonation is a diffusion controlled process, due to the low solubility of ozone and its very fast reaction with lignin. However, whether the process is governed by diffusion on a fibre wall scale (macro diffusion control), on a (micro)fibril scale (micro diffusion control), or by reaction with lignin located on the fibre surface followed by diffusion control is an open question. In order to address this, a 31 kappa Hemlock kraft pulp impregnated with acidified (pH = 2.0) water or acidified (pH = 2.0) ethanol-water (70% w/w) was ozonated at high consistency in a differentially operated packed bed reactor. The cellulose degradation and pulp delignification rates were determined from the change in pulp viscosity and kappa number with ozonation time.

ISSN:0277-3813    

DOI:10.1080/02773819809349573

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Spin-trapping E.S.R. spectroscopy has been used to demonstrate that chlorine radicals (Cl°) were formed in conditions simulating chlorine dioxide (ClO2) bleaching, in presence of iron cations. Chlorine radicals (Cl°) most probably originate from reaction of Fe2+/Fe3+ with HCLO2and with Cl2/ClOH, equally produced when ClO2reacted with a phenolic unit. Adding Fe2+Fe/3+ in a D bleaching stage of a softwood kraft pulp had a detrimental effect on both pulp viscosity and delignification. At the same time AOX and chlorate formation were increased. These effects were in accordance with the proposed mechanism for the formation of Cl° and showed that iron might be potentially harmful, if in a sufficient quantity in pulp or process water.

ISSN:0277-3813    

DOI:10.1080/02773819809349574

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The fractionation of wheat straw was studied using a two-stage process based on an hydrolytic pretreatment followed by alkali delignification. The hydrolytic pretreatment was performed by steam explosion. Straw was steamed at temperatures comprised between 205 and 230°C for 2 min. The steamed straw was washed with hot water to yield a solution rich in hemicellulose-derived mono- and oligosaccharides. The washed fiber was delignified by alkali at 160°C for 60 min using a NaOH load of 20%. The alkali-soluble lignin was recovered by filtration after acidification of the black liquor. The delignified pulp was screened and bleached to produce viscose-grade cellulose. The optimization of the pretreatment conditions led to the following results at a pretreatment severity of log(R0) = 3.80: yield of viscose-grade cellulose = 70% of the potential; lignin recovery = 70% of the Klason lignin present in the untreated straw; hemicellulose sugars = 55% of the potential, recovered as molasses.

ISSN:0277-3813    

DOI:10.1080/02773819809349575

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We studied the effects of the hydroxylation patterns and the degrees of polymerization of condensed tannins on their metal-chelating capacity. These effects were evaluated by the relative stability of aluminium complexes with proanthocyanidin. The relative stability of aluminium complexes was determined qualitatively in methanol solution by the competition with the metallochromic reagent (chrome azurol S). A comparison of these capacities indicates the following results: 1) phenolic hydroxy groups in tannins are essential sites for the chelation with metal. 2) the sites of tannins interacting with metal ions are mainly o-dihydroxyphenyl groups in B-ring. 3) increasing degrees of polymerization of tannins tends to increase the relative stability of condensed tannin-aluminium chelate.

ISSN:0277-3813    

DOI:10.1080/02773819809349576

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Feruloyl esters of ω-hydroxyacids were found in the alcoholysis products of suberin from Quercus suber cork. Their identification was made from mass spectra and by comparison with synthetic model compounds. The co-hydroxyls of suberinic hydroxyacids are thought to be the ester-bonding points between suberin aliphatics and associated aromatics.

ISSN:0277-3813    

DOI:10.1080/02773819809349577

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Two key reactions in the DFRC method have been examined by NMR. Both acetyl bromide (AcBr) derivatization of lignin and Zn reductive elimination of the β-bromo derivatives from lignin were highly selective and essentially quantitative. Treatment with AcBr in acetic acid efficiently converted β-aryl ether substructures of lignins into β-bromo ethers while γ-hydroxy and phenol groups were acetylated; the following Zn step cleaved brominated β-aryl ethers forming the expected cinnamyl acetates. In view of the high selectivity of AcBr reactions with lignin units and the solubilization of lignocellulosic materials, AcBr derivatization of lignins can be used for NMR characterization of whole lignins.

ISSN:0277-3813    

DOI:10.1080/02773819809349578

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

During the last decade, high pH anion exchange chromatography with pulsed amperometric detection (HPAEC/PAC) has gained increasing acceptance as the method of choice for analysis of neutral sugars commonly occurring in woods, pulps, and other lignocellulosics. This paper describes modified chromatographic conditions and discusses other critical factors that improve the precision and efficiency of this application. The method involves a controlled loading of acetate onto the column prior to equilibration with water and injection of sample. In-line solid-phase extraction is used to remove hydrophobic substances that have the potential to foul the analytical column. Critical operational parameters for the successful application of the method include a metal-free flowpath and a consistent application of anions with sample. Resolution of rhamnose is achieved while maintaining the resolution of xylose and mannose. Simplified sample pretreatment allows a ca. five-fold increase in sample through-put compared with gas chromatography of derivatized sugars or to partition chromatography. Run times are less than half those of the widely used hydroxide reverse gradient method for HPAEC/PAD analysis of wood sugars. Long-term system performance data indicate that the method is highly precise and robust. The acetate loading method affords better precision than those of other HPAEC/PAD methods and of gas chromatographic analysis of alditol acetate derivatives by Tappi Method T249 cm−85.

ISSN:0277-3813    

DOI:10.1080/02773819809349579

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor