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The inter- and intramolecular selectivity of the carbonate radical anion in its reactions with lignin and carbohydrates
KTH, Superseded Departments, Chemistry.
2003 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

In the present thesis, the effects of the carbonate radicalanion on lignin and cellulose were investigated.

Kinetic measurements by pulse -radiolysis were performed tostudy the reactions of simple lignin and cellulose models withthe carbonate radical anion. These studies showed that ligninmodel compounds react with the carbonate radical anion by afast one-electron transfer. For carbohydrates, the kineticstudies showed that carbohydrates could be activated towardsoxidation due to deprotonation as pH is increased.Surprisingly, this was also the case formethyl-b-D-glucopyranoside, for which a pKA-value of about 14.4was estimated. Such activation of this cellulose modelcompound, representing intramolecular glucose, implies that thecellulose itself will be activated towards oxidation at highpH. This could lead to lowered selectivity for technicalprocesses requiring high pH.

The mechanism by which the carbonate radical anion reactswith D-glucose in alkaline deoxygenated aqueous solutions wasstudied by means of ã-radiolysis. From the productanalysis it follows that the reaction is initiated by aone-electron transfer reaction between the carbonate radicalanion and the deprotonated D-glucose. A unique end-product ofthis reaction sequence is formic acid. A reaction scheme wasproposed to account for the observed formation of formicacid.

Finally, comparative studies on the degradation effects(decreased viscosity and molecular weight) caused by carbonate(CO3-•) and hydroxyl (HO•) radicals in cotton linterswere performed in oxygenated or deoxygenated water suspensions,using peroxynitrite as the radical precursor. Both radicalscould degrade cotton linters as shown by viscosity and GPC-SECmeasurements. As evidenced by the viscosity measurements, itseems that the presence of oxygen in the cotton linterstreatments increases the cellulose degradation for bothradicals. The degradation of cotton linters depends on theorder of mixing the reactants. These results can be explainedby competitive reactions, i.e. carbonate radicals react withother species than cellulose. For the carbonate radical, 90 %of the viscosity losses could be recovered with a NaBH4treatment before the viscosity measurements. In contrast, therecovery after hydroxyl radical degradation and a subsequentNaBH4 treatment was 40 %. It was concluded that carbonateradicals preferentially oxidizes C2 and C3 positions in thepyranose rings to carbonyl groups, which can induce indirectcleavage of the glucosidic linkages when treated with strongalkali. The proposed oxidation mechanism proceeds via carbonylformations at C2 and C3 positions. In contrast, the hydroxylradical can abstract hydrogen atoms from C1-C6, some of whichmay lead to direct cleavage of the glucosidic linkages.

Place, publisher, year, edition, pages
Stockholm: Kemi , 2003. , 48 p.
Trita-KKE, ISSN 0349-6465 ; 0304
URN: urn:nbn:se:kth:diva-1625ISBN: OAI: diva2:7554
NR 20140805Available from: 2003-09-10 Created: 2003-09-10Bibliographically approved

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