Chemistry of biomass

Lecture 2

Agenda

Cellulose

Hemicellulose

s

Lignin

They are all

POLYMERS

Major carbohydrates (Fisher projection)

CHO

OHH

HHO

HHO

OHH

CH2OH

D-Galactose

CHO

OHH

HHO

OHH

OHH

CH2OH

D-Glucose

CHO

HHO

HHO

OHH

OHH

CH2OH

D-Mannose

OHH

HHO

OHH

CH2OH

CHO

D-Xylose

OHH

HHO

HHO

CH2OH

CHO

L-Arabinose

Major carbohydrates (Haworth)

Hexoses

O OH

CH2OH

OH

HO

OH

-D-Galactopyranose

O OH

CH2OH

OH

OHHO

-D-Glucopyranose

O OH

CH2OH

OHHO

HO

-D-Mannopyranose

PentosesO OH

OH

OH

HO

-L-Arabinopyranose

O OH

OH

OH

HO

-D-Xylopyranose

O

OH

OH

HOH2C OH

-L-Arabinofuranose

HO

Important monosaccharide projections

CHO

OHH

HHO

OHH

OHH

CH2OH

O

H

HO

H

HO

H

HO

OHHH

OH

Haworth Chair ConfigurationNotes

D-glucose

Fisher

α-D-glucopyranose

O

HO OH

OH

OH

CH2OH

α-D-glucopyranose

Cellulose

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.

O

O O

O

O

O

O

O

CH2OH

OHHO

OHHO

CH2OHOH

CH2OH

HO

OH

CH2OH

HO

O

Cellulose

Cellobiose Unit

Cellulose: DP

Degree of Polymerization of Cellulose

DP = molecular weight of cellulose

molecular weight of one glucose unit

Degree of polymerization

Material Degree of polymerizationCotton (unopened) 15,300

Aspen (Hardwood) 10,300

Jack Pine (Softwood) 7900

Bacteria 5000

Sulfite pulp, bleached 1255

Kraft pulp 975

Rayon 305

Notes

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?

O

O O

O

O

O

O

O

CH2OH

OHHO

OHHO

CH2OHOH

CH2OH

HO

OH

CH2OH

HO

O

Cellulose

Cellobiose Unit

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.

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

Cellulosecrystalline versus amorphous*

Cellulose Crystallinity (%)

Cotton Linters 71.3

Bleached Kraft (spruce) 68

Bleached Sulfite (spruce) 67.8

Bleached Kraft (birch) 65.1

Bleached Kraft (bamboo) 59.9

Rayon 45

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

Representation of cell wall components

Cellulose (elementary fibril)

Hemicelluloses

Lignin

Notes

Cellulose in cell walls (1)

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

Cellulose I unit cells

a

b

Notes

Cellulose I bonding

a

b

Notes Hydrogen Bonds

Hydrogen Bonds

v.d. Waals

Bond strength comparison

Linkage Compound EnergykJ/mol

v.d. Waals H2O 0.155

O-H....O H2O 15

O-H....O Cell-OH 28

O-H 460

C-C 347

Notes

Hemicellulose

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

Cellulose/hemicellulose comparison

Hemicellulose FragmentHemicellulose Fragment

Folded Cellulose FragmentFolded Cellulose Fragment

Hemicellulose classifications

Softwood Hemicelluloses» Galactoglucomannan (Mannans)-main» Arabinoglucuronoxylan (Xylans)» Arabinogalactan» Pectins

Hardwood Hemicelluloses» Glucuronxylan (Xylans)-main » Glucomannan

Grasses» Arabinoxylan-main

Softwood Xylans

4--D-Xly-14--D-Xly-14--D-Xly-14--D-Xly4--D-Xly

4-O-Me--D-Glc

-L-Araf

O

O

HO

O

OO

HO

OH

OO

O

OH

OO

HO

OH

O

O

H3CO

HO

OH

COOHH

OH

HO

O

HO

1

31

4

2

Hardwood Xylans

4 - - D - Xly-14 - - D - Xly-14 - - D - Xly-14 - - 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-16

-D-Gal

1

-D-Gal

1

6-D-Gal

1

-D-Gal

1

6-D-Gal

1

-D-Gal

1

6

6 6 6 6

1

R

R = galactopyranose or L-Arabinofuranose or D-glucopyranosyluronic acid

-L-Araf

1

-L-Ara

1

3

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 >3%

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

Lignin

What is holding all these fibers, vessels together in the

biomass? 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

CH2 O

OH

OCH3

COHH

HC

CH2OH

OH CH2OHC

O

H3CO

C O

CH

CH

H

CH

CHOH2

HO

H

CH

OCH3

OH

C

OH2C

CHO

O

C

CH2OHH3C

O

O

COH

O CH

H3C

CH2OH

H

HCOH

1

2

3

4

5

6

7

H

HC CH

O

O CH

CH2O

C

OCH3

O

CHO

H2C

H3C

8

OHC CH CH2OH

CH2OH

O

O

C

OH

H3C9

10

O

HC CH

COHH2

CH2O

CH

O

OH

H3C

11

H3C

12

HO

CH2OHH3C

13

O

C

O

CH

O CH

O

H3C

H

CH3

CH

OH

O

CH

H3C

CH

H2COH

15

16

Carbohydrate

CH2OH

OH

OCH3

HC 14

H2COH

HC

CHO

17

HOCHO

O

C O CH2

H3C18

HCHO

O

H

H3C

19

O

CH

OCH

O

CH

O

COHH2

OH

OCH3

COHHCOHH2

20

H

CH

H2COH

OCH3

O

HC O

C

OCH3

CH

CH

CHO

22

21

O

H2COHCH2

CH2

H

C O

C

OCH3

24

25

26CH

28

27

O

CH2OH

H

CH3

CH

O

O

H2COH

H

H2COH

H3C

H3C

H2COH

O

CH

CH

OHC

O

O

O

H

23

COH

OCH3

Lignin has been described as 3 dimensional chicken wire. Picture taken from Katy’s chicken page.

Lignin for non-chemists

Lignin biosynthesisNomenclature

OH

OCH3

C

C

C

Methoxyl Group

Phenolic Hydroxyl

1

2

3

45

6

Phenylpropane UnitC9 } Common Names

Side Chain

Lignin nomenclature

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

OH

R

-Hydroxyphenyl

OH

R

OCH3

Guaiacyl Syringyl

OH

R

OCH3CH3O

Lignin structureMethoxyl content

Lignin Source Grass Softwood Hardwood

-Coumaryl Alcohol

(No Methoxyl)

10-25% 0.5-3.5% Trace

Coniferyl Alcohol

(1 methoxyl)

25-50% 90-95% 25-50%

Sinapyl Alcohol

(2 Methoxyls)

25-50% 0-1% 50-75%

Common lignin linkages

O

C

C

C

C

O

C

C

C

O C

O

C

C

C

O

C

C

C

O

C

C

C

O C

O

C

C

C

O

C

C

C

O

C

C

C

O

C

C

C

O

C

C

C

O

O

-O-4 -O-4 -1

- 5-5 4-O-5 -5

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

Extractives

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 Extractives are responsible for the characteristic color and

odor of biomass

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

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.

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.

Flavonoids

Serve many roles in plants:» Protection» Coloration» Other unique roles. (Western Hemlock)

O

OH

OH

OH

OH

HO

Catechin

FlavonoidsMedicinal uses

Higher flavonoid content in diet reduces chance of:» Heart Disease

– Strengthen capillaries– Dilates blood vessels

» Stroke» Cancer: all types reduced

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.

NO

O

N

N

N

N

O

O

N

O

O

Nicotine Caffeine

Cocaine

What is the chemical makeup of wood?

0

10

20

30

40

50

60

%

Douglas Fir Redwood Yellow Pine Balsam Fir

Cellulose*Hemicellulose*Lignin*Extractives

* Data for Cellulose, Hemicellulose & Lignin on extractive free wood basis