Reactions under acidic conditions

Ian Suckling APPI 2012

Polysaccharides

• Polysaccharides hydrolysed to constituent sugars under acidic conditions

• Depends on:- physical structure and accessibility (esp. for cellulose)- conformation of sugar constituents- sidechains for hemicelluloses- hydrolysis medium & conditions

• Resulting sugars more stable than under alkaline conditions, but condensation and dehydration can occur

• Acetyl groups hydrolysed under acidic conditions, esp. at elevated temperatures- Released acetic acid lowers pH and can catalyse hydrolysis

Hydrolysis mechanism

O

OR

CH2OH

OH

HO

OH

O

CH2OH

OH

HO

OH

O

OHCH2OH

HOHO

O

OR

CH2OH

OH

HO

OH

H

O

CH2OH

OH

HO

OH

Glucose+

Other products

H

- ROH H2O

- H

- H

Rates of acid hydrolysis

• In practice, heterogeneous hydrolysis rate differences are even greater:- Cellulose 1- Mannan 60- Xylan 60-80- Galactan300

Dehydration and condensation

• Acid hydrolysis can lead to formation of dehydration and condensation products, depending on hydrolysis conditions

OOHOH

HOOH

OOH

CH2OH

OH

HOOH

O CHO

furfural

condensation products

hydroxymethyl furfural

O CHOHOH2CHCOOH +

HOOC

O

levulinic acid

H+, -3H2O

H+, -3H2O

Hydrolysis of xylans

• Arabinofuranose units readily cleaved off softwood xylans • Glycosidic linkages of uronic acid group only partially hydrolysed so

get biuronic acids after hydrolysis• Uronic acid substituents also slow xylan hydrolysis

O

O

O

OHO

O

OH

O

O

O

O

OO

OH OH

OH OH

O

OHHOH2C

OHCOOH

OH

MeOOH

O

O

O

O

OHOH

COOH

OH

MeOOH

HO

Biuronic acid

Reactions of lignin

• Reactions under acidic conditions• Cleavage of - and -ether linkages• Cleavage of lignin-carbohydrate bonds• Release of formaldehyde due to sidechain cleavage• Condensation reactions

OR1

OR

OCH3

O

HOH3CO

Relative rates for hydrolysis of - and -ether lignin models in 0.2M HCl aqueous dioxane at 50ºC

Linkage Relative rate Type R =

-Aryl ether, R1 = H Nonphenolic CH3 1 Phenolic H 12 -Aryl ether, R1 = Aryl Nonphenolic CH3 65 Phenolic H 305

Acidic cleavage of -ether linkages

HC OR1

OCH3

OR

HC OR1

OCH3

OR

CH

OCH3

OR

HC OH

OCH3

OR

CH

OR

OCH3

-R1OH H2O

slow

Condensationproducts

R = HR1 = aryl, alkyl

Acidic cleavage of -ether linkages

HC OH

OR

OCH3

O

HOH3CO

CH

OR

OCH3

O

HOH3CO

HC

OR

OCH3

O

HOH3CO

HC OH

OR

OCH3

O

H3CO

CH2

CHO

OR

OCH3

OR

OCH3

CH2

O

HO

Guaiacyl

HO O

Guaiacyl

O

Guaiacyl

O O

GuaiacylH2C

O

HCHO+H

Hibbert's ketones

H

- H2O

Other reactions under acidic conditions

• Formaldehyde elimination from sidechain

• Condensation

- species that can trap the intermediate carbonium ion (bisulfite, phenols, thioglycolic acid, chloride) reduce condensation

- liberated formaldehyde can also participate in condensations

HO

H3CO

H2COH

OCH3OH

O H

HO

H3CO

OH

OCH3

+

HCHO

HO

H3CO

H3CO

CH3

OCH3

OH

OHHO

H3CO

H3CO

CH3

OCH3

OH

H2C+

1% HCl

Suppressing condensation reactions

• Addition of 2% of phenol and other additives can suppress lignin condensation- 2-naphthol most effective

CH

OH/Lignin

OMe

O

HOH2C

RO

Lignin

MeO

HC

OH/Lignin

OMe

O

HOH2C

Lignin

MeO

+ CH

OH/Lignin

OMe

O

HOH2C

Lignin

MeOOH

MeO

Lignin

Lignin

MeO

HO

H2C

OH/Lignin

OMe

O

CH2OH

CH

OH/Lignin

OMe

O

HOH2C

Lignin

MeO

OH

OH

H+

-ROH

+

30

A

B

C

OH

Lignin

MeO

Solvent pulping

• Separation of woods by treatment with organic solvents- Range of different solvents- Many include acid or alkali to

enhance pulping rates• Examples:

- Allcel – EtOH/water (1:1), ~190ºC, ~60 min

- Acetic acid/water pulping- Organocell – NaOH,

methanol, catalytic AQ• Requires v. efficient solvent

recovery

Vertichem process – an example