Lecture_6_Wood_Chemistry.ppt

WOOD CHEMISTRY: ?What is wood made of?? Here?s a video clip from Glasgow University about Wood Anatomy and Wood Chemistry http://video.google.com/videoplay?docid=3352818558015705209&hl=en Wood Macroscopic Structure Outer Bark Cambium Xylem Phloem (inner bark) Heartwood Sapwood Pith Annual Rings Earlywood Latewood Knot Wood Microscopic Structure Imagine that wood is made up of millions and millions of toilet paper rolls glued together. These rolls are the fibers that will make paper. Chemical Composition ~50% Cellulose ~25% Hemicellulose ~25% Lignin Variable amounts of extractives 3 Main Wood Chemicals Cellulose Hemicelluloses Lignin They are all POLYMERS What is a polymer? Polymers are made by stringing together a bunch of little molecules to make a big molecule All plastics are polymers Polymers can be linear (long strings) planar (big sheets) three dimensional (like lignin) A bowling ball is one big three dimensional polymer Source: World Book Encyclopedia THE GRADUATE Images from D. Briggs Hardwood vs Softwood Fibers White Pine Red Oak Fiber Structure Lumen Primary Wall Middle lamella (almost all lignin) { Secondary Wall Woody cells consist of several different layers The area between cells is known as the middle lamella The lignin content lowers through the cell. Douglas fir fiber with spiral thickening How is the cell wall put together? Cell wall is assembled by gluing together a bunch of very small fibers called macrofibril Same technology used to produce composite materials today The glue holding the macrofibrils together is lignin Macrofibrils are made up of microfibrils which in turn are made up of cellulose and hemicellulose polymers The glue holding all this together is lignin Representation of Cell Wall Components Cellulose Hemicelluloses Lignin Wood vs. Rice Krispies Treats Wood Ingredients: Cellulose, Hemicellulose, Lignin, Extractives Rice Krispies Treats Ingredients: Rice Krispies, Rice Krispies dust (there?s ALWAYS dust in the bag), Marshmallows, Butter, some flavoring and preservatives. The Perfect Recipe What is Cellulose? Cellulose is straight chain polymer. In bead terms, imagine a very very long straight string of beads with 2 ends and no branching points. In wood, cellulose chains contain typically 10,000 glucose molecules?quite a long string of beads. Source: World Book Encyclopedia Cellulose microfibrils Microfibril diameter is about 1/100,000 mm http://www.dailymotion.com/video/x4mu6i_zoom-into-the-cellulose-of-wood_tech Cellulose For Chemists Very long straight chain polymer of glucose: approximately 10,000 in a row in wood. Cotton is nearly pure cellulose. Cellulose molecules link up in bundles and bundles of bundles and bundles of bundles of bundles to make fibers Cellulose forms tight bundles which are very resistant to chemical attack What are Hemicelluloses? Hemicelluloses are also sugar polymers but different from cellulose because they are: Made up glucose and other sugars. Contain some molecules other than sugars. Branched little polymers The beads have Y?s in them Much smaller than cellulose as they are made up of between 50-300 sugars There are lots of varieties of hemicelluloses. Not very resistant to chemical attack ? many easily break down to simple sugars Hemicelluloses for Chemists Branched little uncolored sugar polymers (~ 50 to 300 sugar units) Composition varies between wood species 5 carbon sugars: xylose, arabinose 6 carbon sugars: mannose, galactose, glucose Uronic Acids: galacturonic acid, glucuronic acid Acetyl and methoxyl groups (acetic acid & methanol) Cellulose/Hemicellulose Comparison Hemicellulose Fragment Folded Cellulose Fragment What is holding all these fibers together in the tree? Lignin Three dimensional polymer No sugars in it Nature?s glue ? very similar to phenolic resin used in plywood. Holds cellulose and hemicelluloses together Second most plentiful natural material Must be removed or weakened to separate fibers; turn wood to pulp Dark in nature ? especially after reacting with alkali ? must be de-colored or removed to bleach pulp Lignin for Chemists Lignin for non-chemists Lignin has been described as 3 dimensional chicken wire. Picture taken from Katy?s chicken page. Representation of Cell Wall Components Cellulose Hemicelluloses Lignin Representation of Cell Wall Components Extractives The term extractives refers to a group of unique chemical compounds which can be removed from plant materials through extraction with various solvents Typically these chemicals constitute only a small portion of the tree (<5%) In some tropical species this can be as high as 25% Extractives are produced by plants for a variety of uses The most common is protection Extractives can cause serious problems for processing Pitch is a term which is often used when describing some groups of extractives Extractives are responsible for the characteristic color and odor of wood Chemical Composition of Wood: Hardwoods * Data for Cellulose, Hemicellulose & Lignin on extractive free wood basis Chemical Composition of Wood: Softwoods * Data for Cellulose, Hemicellulose & Lignin on extractive free wood basis You are starving in the woods when a little elf approaches and ask if you would like to eat one of the following wood compounds ? which do you choose? A. Lignin B. Cellulose C. Hemicellulose D. None of above ? elves are notably mischievous and are not to be trusted Sugars What is a sugar: polyhydroxy aldehydes or ketones (aldoses or ketoses) of 3-9 carbons. Wood Sugars: Glucose Hexose (6 carbons) Glucose is the by far the most abundant wood monosaccharide (cellulose). A small amount can also be found in the hemicelluloses (glucomannans) Sugar we burn in our bodies for energy * Fructose ? ?Fruit Sugar? 6 carbon sugar but with a ketone group instead of an aldahyde They are found in extremely limited amounts in wood but are found in larger amounts in some plants. Roughly twice as sweet as glucose * Fructose Wood Sugars: Xylose Five carbon sugar Xylose is the major constituent of xylans (a class of hemicelluloses). 3-8% of softwoods 15-25% of hardwoods You (along with many yeast and bacteria) don?t metabolize (burn) five carbon sugars * Sucrose ? Table Sugar Glucose + Fructose Produced from sugar cane or sugar beets Sweetness in between fructose and glucose Relative Sweetness Starch Isolated from many different biomass sources: potatoes, corn, etc. Largest use in food applications. Starch is also used in many non-food applications. Starch in Plants Starch serves as an energy reserve in plants. High concentrations of starch are found in seeds, bulbs, and tubers. Plants high in starch include: Potatoes (1/5) Rice (4/5) Rice, wheat, and corn (3/4) Oats (2/3) Starch Starch is a polymer of glucose ? just like cellulose BUT has a much more reactive structure Glucose can be easily liberated from starch by enzymes Corn and wheat are very cheap sources of glucose Fun with sugar ? two ways to make a pile of $$$$$$ 1. Develop a cheaper source of sweetener for soda pop and twinkies Fun with sugar ? two ways to make a pile of $$$$$$ 1. Develop a cheaper source of sweetener for soda pop and twinkies Glucose Fructose Enzymes Fun with sugar ? two ways to make a pile of $$$$$$ http://www.youtube.com/watch?v=hYiEFu54o1E http://www.youtube.com/watch?v=KVsgXPt564Q Fun with sugar ? two ways to make a pile of $$$$$$ 2. Develop a sweetener with fewer calories Xylose Xylitol Sweet but not metabolized