Chemistry of biomass

1
Chemistry of biomass

• Lecture 2

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Agenda

• Cellulose
• Hemicelluloses
• Lignin

They are all POLYMERS
3
Major carbohydrates (Fisher projection)
D-Xylose
L-Arabinose
D-Galactose
D-Glucose
D-Mannose
4
Major carbohydrates (Haworth)
Hexoses
HO
Pentoses
5
Important monosaccharide projections
D-glucose
a-D-glucopyranose
a-D-glucopyranose
Haworth
Chair Configuration
Fisher
Notes
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Cellulose
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Cellulose the basics

• Linear polymer made up of ?-D glucopyranose units
  linked with ? ??? glycosidic bonds.
• Repeating unit glucose (cellobiose)
• Glucopyranose units in chair form - most
  thermodynamically stable. Only 1 or less in
  other forms.

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Cellulose DP
Degree of Polymerization of Cellulose
molecular weight of cellulose
DP
molecular weight of one glucose unit
9
Degree of polymerization
Notes
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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 dont stir the spaghetti when
  cooking?

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Amorphous cellulose

• A portion of the cellulose in the cell wall can
  be though 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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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

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Cellulosecrystalline versus amorphous
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How is the cell wall put together?

• Cell wall is assembled by gluing together a bunch
  of very small fibers called macrofibril
• 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

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Representation of cell wall components
Cellulose (elementary fibril)
Hemicelluloses
Lignin
Notes
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Cellulose in cell walls (1)
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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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Cellulose I unit cells
Notes
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Cellulose I bonding
Hydrogen Bonds
v.d. Waals
b
a
Notes
Hydrogen Bonds
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Bond strength comparison
Notes
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Hemicellulose
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Hemicellulose-general information

• Cell wall supporting components
• 27-30 of wood
• 27 softwoods
• 30 hardwoods
• 30 agricultural biomass
• Short branched polymers
• 50-300 DP
• In wood they are not crystalline
• Very accessible to chemicals
• Very reactive

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Cellulose/hemicellulose comparison
Folded Cellulose Fragment
Hemicellulose Fragment
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Hemicellulose classifications

• Softwood Hemicelluloses
• Galactoglucomannan (Mannans)-main
• Arabinoglucuronoxylan (Xylans)
• Arabinogalactan
• Pectins
• Hardwood Hemicelluloses
• Glucuronxylan (Xylans)-main
• Glucomannan
• Grasses
• Arabinoxylan-main

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Softwood Xylans
2
3
1
1
4
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Hardwood Xylans
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Arabinogalactan
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Starch in plants

• 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 70-80 of certain tubers
  and seeds.
• Wood contains minor amounts of starch in the form
  of granules in living parenchyma cells.
• Typical amounts 0.2-0.6 of total wood
• Sapwood gt3

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Chemical composition of starch

• Plants contain two types of starch, linear
  (helix) amyloses and branched amylopectins.
• 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

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

• Lignin
• Three dimensional polymer
• No sugars in it
• Natures 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

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Lignin for chemists
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Lignin for non-chemists

• Lignin has been described as 3 dimensional
  chicken wire.

Picture taken from Katys chicken page.
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Lignin biosynthesisNomenclature
Side Chain

Phenylpropane Unit C9
Common Names
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Lignin nomenclature

• Once incorporated into lignin, the ring
  structures of the precursors are given these
  names.

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Lignin structureMethoxyl content
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Common lignin linkages

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

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Extractives
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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 (lt5)
• 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
• Extractives are responsible for the
  characteristic color and odor of biomass

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Tree extractives (1)

• 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
• The amount of extractives in wood can range from
  1-20 (species, position in the tree, season,
  geographical location)
• More in heartwood

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Tree extractives (2)

• Extractives add significant properties to wood
• Color
• Odor
• Density
• These compounds are typically present in very
  limited amounts but still affect the wood
  properties greatly.

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The fragrance of a tree

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

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Flavonoids

• Serve many roles in plants
• Protection
• Coloration
• Other unique roles.

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FlavonoidsMedicinal uses

• Higher flavonoid content in diet reduces chance
  of
• Heart Disease
• Strengthen capillaries
• Dilates blood vessels
• Stroke
• Cancer all types reduced

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Alkaloids

• These nitrogen containing compounds are found in
  a variety of different plants.
• Located in the leaves, fruits, and bark.
• 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.

Caffeine
Nicotine
Cocaine
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What is the chemical makeup of wood?
Data for Cellulose, Hemicellulose Lignin on
extractive free wood basis