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Biomass Fundamentals Modules 12 : Cellulose & Hemicelluloses

Biomass Fundamentals Modules 12 : Cellulose & Hemicelluloses. A capstone course for BioSUCCEED : Bio products S ustainability: a U niversity C ooperative C enter of E xcellence in ED ucation. The USDA Higher Education Challenge Grants program gratefully acknowledged for support.

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Biomass Fundamentals Modules 12 : Cellulose & Hemicelluloses

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  1. Biomass FundamentalsModules 12: Cellulose & Hemicelluloses A capstone course for BioSUCCEED: BioproductsSustainability: a University Cooperative Center of Excellence in EDucation The USDA Higher Education Challenge Grants program gratefully acknowledged for support

  2. This course would not be possible without support from: USDA Higher Education Challenge (HEC) Grants Program www.csrees.usda.gov/funding/rfas/hep_challenge.html

  3. Cellulose b-D-anhydroglucopyranose units linked by (1,4)-glycosidic bonds

  4. Cellulose b-D-anhydroglucopyranose units linked by (1,4)-glycosidic bonds

  5. Why the Cellulose Story is so Convoluted? Polymorphism or allotropy refers to the existence of more than one crystalline forms differing in physical and chemical properties. Four major polymorphs of cellulose have been reported • Cellulose I, • Cellulose II, • Cellulose III, • Cellulose IV

  6. Early Models of Cellulose Morphology After Polymers Before Polymers Micellar Structure Fringed Micellar Structure Alignment of chains followed by folding; entropically not very favourable Blocks of material mixed with amorphous material

  7. More Recent Models of Cellulose Morphology Microfibrils exist as discrete crystalline regions of >600Å, separated by less ordered amorphous domains. No chain folding

  8. What is Happening During Acid Hydrolysis? The non-ordered domains of the long Microfibrils are degraded leaving behind the discrete crystallites

  9. Tc Tc Tc Tc Packing of the Cellulose Microfibrils in Cell Wall R R R R

  10. Tc Tc Tc Tc Tc Tc Tc or R R R R R R

  11. First Reported Unit-Cell for Cellulose Meyer-Mark-Misch- Chains run anti-Parallel within the unit cell. Meyer et al. (1929)

  12. Gardner & Blackwell Chains run Parallel within the unit cell. Gardner and Blackwell (1974) The unit cell parameters were almost double that of the Meyer-Mark and Misch unit cell. a = 16.34 Å, b = 15.72Å, c = 10.38Å (fiber axis), angle β = 97o.

  13. Blackwell Unit-cell Parallel chain orientation Accepted today Meyer-Misch Unit-cell Anti-parallel chain orientation In summary for Native Cellulose I

  14. How does one go about unequivocally confirming chain orientation? • Nature has provided a way to do so. • The fact that cellulose has two distinct ends a reducing end group (aldehyde) and a non-reducing end group allows for some creative chemistry to be done

  15. R Staining of the Chain Ends NR Hieta et al., Biopolymers, 23, 1807,1984

  16. Elegant Proof of Chain Orientation by selective staining of the reducing end-groups For parallel model one expects the black dots to preferentially be on one end of microfibrils. Kuga et al. (1984) and Chanzy

  17. The intra-molecular hydrogen-bonds are responsible for the stiff and rigid nature of the cellulose molecule. Cellulose I Intra-molecular Hydrogen-Bonding • Due to the equatorial orientation of the hydroxyl groups and its linear structure, cellulose molecules have a strong tendency to form intra- and inter-molecular hydrogen-bonds.

  18. Cellulose I Intra-molecular Hydrogen-Bonding • Two kinds of such bonds form within the same chain: • C3-OH with endocyclic oxygen • C6 –OH (primary) with the C2-OH

  19. Cellulose I Inter-molecular Hydrogen-BondingHowever, one kind of H-bond forms between neighboring chains • C3 OH and C6 OH

  20. 1/4 Cellulose :Nature Working Across a Length Scale >1010! Cellulose nanofiber bundles 6 assembly proteins (rosette) which produce cellulose nanofibers ~28nm C. Haigler & L. Blanton

  21. Cellulose II, Another from of Cellulose • Created from From cellulose I • Via Mercerization - 17-20% NaOH • And Regeneration - precipitated from solution • Two-chain unit-cell • Anti-parallel chain orientation • Of Lower crystallinity than Cellulose I

  22. Cellulose I Cellulose II Mercerization

  23. Why the Transformation? • Mercerization; Heterogeneous alkali swelling • Several mechanisms proposed • Conformational change • “Bent” cellulose I to “twisted & bent” cellulose II • Recrystallization of cellulose II on cellulose I • “shish-kebab” structure • Chain-folding • Progressive ‘shifting’ of sheets or chains

  24. Why the Tansformation contd. • Since the C6 hydroxyl group is involved in two secondary valence interactions, it is precluded from interacting with molecules in neighboring planes (above and below). • Therefore cellulose has a sheet-like structure with only weak van der Waals forces holding the sheets together . These sheets fall apart during mercerization • New orientations and H-bonds may now form • Along different planes C2 OH may now H-bond

  25. Cellulose II Hydrogen-bonding New H bonds between the C2 OH groups in neighbouring planes can set in after mercerization. Plane 1 Plane 2 Plane 3

  26. Hemicelluloses Structurally, hemicelluloses are co-polymers of two or more sugars and sugar acids • glucose, mannose, galactose, xylose, arabinose and 4-O-methylglucuronic acid They are of low DP 120 - 200 with short branching chains, making them amorphous heteropolysaccharides

  27. Hemicelluloses These are Heteropolysaccharides Supporting material in cell walls that vary from plant to plant and from one plant part to another. In woody plants, there are two basic types • D glucomannans • D glucuronoxylans The composition and amount of each is species dependent • Softwood vsHardwood

  28. Softwood Hemicelluloses (major) The principle hemicellulose of softwoods is the galactoglucomannans (~ 20% of woody material) Note: Galactose C4 OH is axial Mannose C2 OH is axial Alternating Glucose & Mannose along the main chain; Galactose branches off; Random acetates at C5 & C3 of main chain Note β-1-4 links along the main chain

  29. Softwood Hemicelluloses (major) The principle hemicellulose of softwoods is the galactoglucomannans (~ 20% of woody material) They are subdivided as : High Galactose content: Galactose 1/Glucose 1/Mannose/4 Low Galactose content Galactose 0.1/Glucose 1/Mannose/3

  30. Softwood Hemicelluloses (minor) The minor hemicellulose of softwoods is the Arabinoglucuronoxylans (~5-10% of wood) Note: Xylan has no C6 primary OH Note: Arabinose is a Furan; 5 member ring sugar structure with a C4 primary OH Note β-1-4 links along the main chain Xylanose linked along the main chain; arabinose and glucuronic acid branches off

  31. Harwood Hemicelluloses (major) The principle hemicellulose of hardwoods is the glucuronoxylans (15-30% of woody material) Note β-1-4 links along the main chain

  32. Hardwood Hemicelluloses (minor) The minor hemicellulose of hardwoods is the glucomannans (2-5% of wood) Note β-1-4 links along the main chain

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