Freitag, 24. Mai 2013, 14:15 - 15:45 iCal
Towards a better understanding of cellulose swelling, dissolution and regeneration at the molecular level
Gastvortrag von Univ.-Prof. Dr. Antje Potthast und Univ.-Prof. Thomas Rosenau (Universität für Bodenkultur Wien)
Universität Wien, Chemische Institute, Seminarraum 2 beim Dekanat, Raum 2124, 1. Stock
Währinger Straße 42, 1090 Wien
Seminar, Workshop, Kurs
The exact structure of the hydrogen bond networks and the changes of these networks upon swelling and dissolution processes are current ‘hot topics’ in cellulose research. H-bonds are responsible for the allomorphism of cellulose, for the typical properties of cellulose, and for reactivity and chemical behavior. The use of isotopic labeling in combination with modern NMR techniques and X-ray crystal structure analysis is a powerful approach to obtain solid state and gel structural data of cellulose and cellulose model compounds, so that we now come closer to an understanding of cellulose swelling and dissolution on a molecular level, and might even tackle the old and unanswered question about the special nature of cellulose solvents.
The cellulose model compound methyl 4’-O-methyl-beta-D-cellobioside was the first cellulose fragment analogue found to form two distinct crystal phases, by analogy to the cellulose I and II allomorphs. With the 13C-perlabeled form of this compound, novel solid-state NMR experiments that were based on the high degree of isotopic enrichment (>99%) became possible. Protons in hydrogen bonds are detected through the two carbons that are bridged by this proton. The cleavage and re-formation of the complex hydrogen bond network became accessible to detailed analysis for the first time. In the next steps, both 13C-perlabeled cellulose (cellulose II) synthesized according to the cationic ring-opening polymerization approach from 13C6-glucose and 13C-enriched bacterial cellulose (cellulose I) were subject to similar experiments.
We selected the following cellulose solvents for our studies, which were synthesized in perdeuterated and 15N-labeled form: NMMO, DMAc, and BMIM acetate. 15N-labeling allows measuring the defined distance between solvent and the respective cellulose (model) carbon, and thus monitoring approach and action of the solvent from the viewpoint of the solute.
The studies showed that swelling is a reversible process of 3-4 stages, connected with cleavage of hydrogen bonds mainly to/from OH-6 and OH-2. Dissolution, by contrast, is irreversible and involves in addition H-bonds to/from OH-3. The solvents can be distinguished by (1) the order in which specific H-bonds are broken, (2) the number of distinguishable swelling stages, (3) the number of solvent molecules per anhydroglucose unit, and (4) the solvent distance to the different AGU carbons. In addition to common O-H hydrogen bonds, cellulose solvents also form non-conventional C-H hydrogen bonds involving selectively the C-1 and C-3 positions along the cellulose chain. This C-H hydrogen bond formation might be a prerequisite to cellulose dissolution. In solution (confirmed so far for the solvents DMAc, NMMO and BMIM acetate) the cellulose molecules are surrounded by a layer of tightly bound solvent molecules which are not undergoing dynamic exchange, comparable to primary “solvent shells” known in inorganic chemistry.
All these molecular level data provide a consistent picture of the molecular mechanisms of swelling and dissolution of cellulose and cellulosic model compounds, which will be discussed in the presentation.
Veranstalter
Institut für Physikalische Chemie
Kontakt
Univ.-Prof. Dr. Wolfgang Kautek
Universität Wien
Institut für Physikalische Chemie
01 4277 52470
wolfgang.kautek@univie.ac.at
Erstellt am Mittwoch, 15. Mai 2013, 18:31
Letzte Änderung am Freitag, 17. Mai 2013, 07:41