Chemistry of biomass Lecture 2

Agenda l Cellulose l Hemicelluloses l Lignin They are all POLYMERS

Major carbohydrates (Fisher projection) D-GalactoseD-GlucoseD-Mannose D-Xylose L-Arabinose

Major carbohydrates ( Haworth) Hexoses Pentoses HO

Important monosaccharide projections HaworthChair Configuration Notes D-glucose Fisher α- D-glucopyranose

Cellulose

Cellulose: the basics Linear polymer made up of  -D glucopyranose units linked with  glycosidic bonds. l Repeating unit = glucose (cellobiose) l Glucopyranose units in chair form - most thermodynamically stable. Only 1% or less in other forms.

Cellulose: DP Degree of Polymerization of Cellulose DP = molecular weight of cellulose molecular weight of one glucose unit

Degree of polymerization Notes

Is cellulose like spaghetti? l 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?

Amorphous cellulose l A portion of the cellulose in the cell wall can be though of as flexible spaghetti. This is amorphous cellulose. l Every different cellulose preparation has different percentages of amorphous and crystalline cellulose (see next slide). l These 2 forms of cellulose have different properties and reactivities.

Cellulose: physical properties l 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

Cellulose crystalline versus amorphous*

How is the cell wall put together? l Cell wall is assembled by gluing together a bunch of very small fibers called macrofibril l The glue holding the macrofibrils together is lignin l 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 (elementary fibril) Hemicelluloses Lignin Notes

Cellulose in cell walls (1)

Types of cellulose l Cellulose I: Native cellulose (cellulose as found in nature. l Cellulose II: Native cellulose which has been soaked in alkali or regenerated cellulose. Large structural changes have occurred in the molecule l Cellulose III or IV: Forms of cellulose which have been treated with various reagents

Cellulose I unit cells a b Notes

Cellulose I bonding a b Notes Hydrogen Bonds v.d. Waals

Bond strength comparison Notes

Hemicellulose

Hemicellulose-general information l Cell wall supporting components l 27-30% of wood » ~27% softwoods » ~30% hardwoods » ~30% agricultural biomass l Short branched polymers » DP l In wood they are not crystalline » Very accessible to chemicals » Very reactive

Cellulose/hemicellulose comparison Hemicellulose Fragment Folded Cellulose Fragment

Hemicellulose classifications l Softwood Hemicelluloses » Galactoglucomannan (Mannans)-main » Arabinoglucuronoxylan (Xylans) » Arabinogalactan » Pectins l Hardwood Hemicelluloses » Glucuronxylan (Xylans)-main » Glucomannan l Grasses » Arabinoxylan-main

Softwood Xylans  4-  -D-Xly  -1  4-  -D-Xly  -1  4-  -D-Xly  -1  4-  -D-Xly  4-  -D-Xly   4-O-Me-  -D-Glc         -L-Araf 

Hardwood Xylans  4 -  - D - Xly  -1  4 -  - D - Xly  -1  4 -  - D - Xly  -1  4 -  - D - Xly    R  R = Acetyl 4 - O - Me -  - D - Glc    

Arabinogalactan  3-  -D-Gal  -1  3-  -D-Gal  -1  3-  -D-Gal  -1  3-  -D-Gal  -1  3-  -D-Gal  -1 6  -D-Gal   1  1 6  1  1 6  1   1 R R = galactopyranose or L-Arabinofuranose or D-glucopyranosyluronic acid  -L-Araf  1  -L-Ara   1 3

Starch in plants l Starch serves as an energy reserve in plants. » High concentrations of starch are found in seeds, bulbs, and tubers. » Starch can be as high as % of certain tubers and seeds. l Wood contains minor amounts of starch in the form of granules in living parenchyma cells. » Typical amounts: % of total wood » Sapwood >3%

Chemical composition of starch l Plants contain two types of starch, linear (helix) amyloses and branched amylopectins. l The amounts of each of these starch types present is plant dependent. » Typical amounts are 25% amylose, 75% amylopectin » Mutant species can have from 50-90% amylose

Lignin

What is holding all these fibers, vessels together in the biomass? l 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

l Lignin has been described as 3 dimensional chicken wire. Picture taken from Katy’s chicken page. Lignin for non-chemists

Lignin biosynthesis Nomenclature Phenylpropane Unit C9 } Common Names Side Chain

Lignin nomenclature l Once incorporated into lignin, the ring structures of the precursors are given these names.

Lignin structure Methoxyl content

Common lignin linkages l The linkages shown on the right are those formed in dehydrogenation polymers and also found in wood.

Extractives

l The term extractives refers to a group of unique chemical compounds which can be removed from plant materials through extraction with various solvents l Typically these chemicals constitute only a small portion of the tree (<5%) » In some tropical species this can be as high as 25% l Extractives are produced by plants for a variety of uses » The most common is protection l Extractives can cause serious problems for processing l Extractives are responsible for the characteristic color and odor of biomass

Tree extractives (1) l Besides the big three wood compounds, trees contain other compounds that serve a variety of functions including: » Protection (from insects, animals, and rot). » Attractants (flowers, fruits) » Food storage l The amount of extractives in wood can range from 1- 20% (species, position in the tree, season, geographical location) l More in heartwood

Tree extractives (2) l Extractives add significant properties to wood: » Color » Odor » Density l These compounds are typically present in very limited amounts but still affect the wood properties greatly.

The fragrance of a tree l Each tree has a unique fragrance. » Some have strong fragrances than others like Cedar. » Some have only light odor. l The aroma is due to volatile compounds produced by the tree (the odor chemicals become gases easily). l These chemicals can be isolated and sold.

Flavonoids l Serve many roles in plants: » Protection » Coloration » Other unique roles.

Flavonoids Medicinal uses l Higher flavonoid content in diet reduces chance of: » Heart Disease –Strengthen capillaries –Dilates blood vessels » Stroke » Cancer: all types reduced

Alkaloids l These nitrogen containing compounds are found in a variety of different plants. » Located in the leaves, fruits, and bark. l You are all aware of the alkaloids shown on this page you probably have never seen their structures. These are typically found in small amounts in plants but are worth large sums of cash. Nicotine Caffeine Cocaine

What is the chemical makeup of wood? * Data for Cellulose, Hemicellulose & Lignin on extractive free wood basis