Wood Chemistry PSE 406/Chem E 470 Cellulose Lecture 6 PSE 406 - Lecture 6

Agenda Amorphous versus crystalline cellulose Cellulose structural considerations Cellulose I Orientation Bonding Cellulose II Cellulose physical properties PSE 406 - Lecture 6

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 406 - Lecture 6

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. PSE 406 - Lecture 6

Cellulose Crystalline versus Amorphous* * See Sjostrom pg 13 for a drawing PSE 406 - Lecture 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 10-30 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). PSE 406 - Lecture 6

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 PSE 406 - Lecture 6

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 PSE 406 - Lecture 6

PSE 406 - Lecture 6

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 PSE 406 - Lecture 6

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 PSE 406 - Lecture 6

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 PSE 406 - Lecture 6

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 PSE 406 - Lecture 6

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 PSE 406 - Lecture 6

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 PSE 406 - Lecture 6

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 PSE 406 - Lecture 6

Cellulose II Structure b 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 PSE 406 - Lecture 6

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 PSE 406 - Lecture 6

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 8.5 -12% NaOH Others PSE 406 - Lecture 6

Other Cellulose Information “Cellulose” that is insoluble in 18% NaOH DP>200 -Cellulose Material that precipitates after 18% NaoH neutralized DP 10-200 -Cellulose Remaining material, DP<10 PSE 406 - Lecture 6