1. Wood Chemistry PSE 406/Chem E 470
Cellulose
Lecture 6
PSE Lecture 6

2. Agenda Amorphous versus crystalline cellulose
Cellulose structural considerations
Cellulose I
Orientation
Bonding
Cellulose II
Cellulose physical properties
PSE Lecture 6

3. Is Cellulose Like Spaghetti?
In the woody cell wall, exactly what is the cellulose doing?
Is cellulose like uncooked spaghetti? i.e. random orientation of rigid cellulose chains.
Is cellulose like cooked spaghetti? i.e random orientation of flexible cellulose chains
Or is cellulose like those clumps of spaghetti you get when you don’t stir the spaghetti when cooking?
PSE Lecture 6

4. Amorphous Cellulose
A portion of the cellulose in the cell wall can be thought of as flexible spaghetti. This is amorphous cellulose.
Every different cellulose preparation has different percentages of amorphous and crystalline cellulose (see next slide).
These 2 forms of cellulose have different properties and reactivities.
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5. Cellulose Crystalline versus Amorphous*
* See Sjostrom pg 13 for a drawing
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6. Cellulose Structural Considerations
If you look at a picture of a cell wall  you can see what looks like threads. These are crystalline bundles of up to 2000 cellulose molecules known as microfibrils . These can range in diameter from nm.
There are theories that the microfibrils are made up of smaller units known as elementary fibrils with diameters of 2-4 nm.
This is an electron microscopic image of a shadowed preparation of cellulose microfibrils from green algae (D. G. ROBINSON, 1986).
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7. Representation of Cell Wall Components
Cellulose
Hemicelluloses
Lignin
This drawing by Fengel (1971) represents a cross section of a plant cell wall containing a series of cellulose molecules encased in hemicelluloses and lignin. In this model, each of the white boxes represents a cellulose elementary fibril with a width of 30 angstroms. As previously discussed, the existence of elementary fibrils is still a matter of debate. The elementary fibrils bundle together to form microfibrils with widths of 120 angstroms. The microfibrils bundle together to form fibrils of width of 2000 angstroms. A review of the current literature on this subject shows a large number of different opinions on this structure.
Notes
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8. Cellular Microscopic Structure
Woody cells consist of several different layers
The area between cells is known as the middle lamella
Very high lignin content
The lignin content lowers through the cell
The idea in chemical pulping is to :
Release the fibers by removing the lignin in the ML
Remove color by removing lignin in the rest of the cell
Cote 1967
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9. PSE Lecture 6

10. Types of Cellulose
Cellulose I: Native cellulose (cellulose as found in nature.
Cellulose II: Native cellulose which has been soaked in alkali or regenerated cellulose. Large structural changes have occurred in the molecule
Cellulose III or IV: Forms of cellulose which have been treated with various reagents
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11. Cellulose I Unit Cells b a Notes PSE 406 - Lecture 6
Elementary fibrils are theorized to contain 36 cellulose molecules (in the cross section). The crystalline structure of these cellulose bundles have been characterized using x-ray diffraction analysis and other techniques. In the figure above, you can see a cross section of 5 cellulose I chains. Remember, cellulose I is native cellulose (as found in the tree). This view shows the a-b plane. The cellulose chains are traveling in the c direction.
Notes
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12. Cellulose I Orientation
By looking at the a-c plane, you can see that the cellulose chains are parallel and oriented (direction of reducing end) in the same direction. The center chain of the 5 chains (in red) is off set slightly from the other chains. a c
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13. Cellulose I Bonding b a Hydrogen Bonds v.d. Waals Notes Hydrogen Bonds
In Cellulose I, adjacent cellulose chains in the a direction are held together through hydrogen bonding. We will see how this bonding occurs in a future slide. There is no hydrogen bonding in the b direction. Instead, the cellulose chains are held in place through the much weaker van der Waal forces.
Notes
Hydrogen Bonds
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14. Bond Strength Comparison
This chart shows the bond energies of different types of bonds. You can see from the C-C and O-H bonds that covalent linkages are by far the strongest bonds in this chart. The hydrogen bonds between cellulose and water and cellulose and another cellulose molecule are over 10 times weaker. Van der Waal forces are over 1000 times weaker. These forces, however, are enough to hold the cellulose in place. In the next slide, you will see how water molecules hydrogen bond with cellulose.
Notes
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15. Hydrogen Bonds in Cellulose I
Intramolecular Bonds
O(6) to O(2)H
O(3)H to ring oxygen
Intermolecular Bonds
O(3) to O(6)H
These Bonds in the AC Plane. Bonding in the b plane through van der Waals forces* c
Both intramolecular (same molecule) and intermolecular bonding (between molecules) has been identified for cellulose molecules in the a-c plane. In the above slide, these hydrogen bonds are listed . This listing shows which oxygens are bound to which hydrogens. For example, O(6) to O(2)H under intramolecular bonding means that the oxygen on the # 6 carbon is bound to the hydrogen on the hydroxyl group on the # 2 carbon.
Notes a
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16. Cellulose & Water Notes PSE 406 - Lecture 6
Besides hydrogen bonding between molecules, cellulose will also form hydrogen bonds with other molecules such as water. In the above slide, 2 cellulose molecules are linked to one another through hydrogen bonding through a series of water molecules. Addition of water to cellulose will result in this type of bonding and the swelling of the cellulose fibers.
Notes
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17. Cellulose II Structureb
The cellulose molecules in cellulose II are aligned in the a-b plane slightly different than in cellulose I. Unlike cellulose I, intermolecular hydrogen bonding occurs between cellulose molecules in the a-b plane between the corner chains and the center chain. Bonding occurs through O(2) of one molecule to O(2)H of a separate molecule and from O(3)H of one to the O(6) of another. a
Notes
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18. Cellulose II Structure
From the a-c plane it is possible to see that although the cellulose molecules are parallel in Cellulose II, the orientation of the center cellulose molecule (in red) is opposite to the corner cellulose molecules. The reducing ends are opposite.
In the a-c plane, the intramolecular and intermolecular bonding is the same as it is in cellulose I.
Intramolecular links: O(6) to O(2)H and O(3) to the ring oxygen
Intermolecular links: O(3) to O(6)H a c
Notes
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19. Cellulose Physical Properties
Sorptive Properties
Crystalline cellulose does not dissolve in most solvents
Molecular length
Inter molecular bonding
Amorphous regions have large number of hydrogen bonding sites available
Cellulose can absorb large amounts of water
Fully hydrated cellulose very flexible
Dry cellulose inflexible and brittle
Swelling of cellulose
% NaOH
Others
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20. Other Cellulose Information
“Cellulose” that is insoluble in 18% NaOH
DP>200
-Cellulose
Material that precipitates after 18% NaoH neutralized DP
-Cellulose
Remaining material, DP<10
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