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

Rates of acid hydrolysis In practice, heterogeneous hydrolysis rate differences are even greater:  Cellulose1  Mannan60  Xylan60-80  Galactan300

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

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 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

Acidic cleavage of  -ether linkages

Acidic cleavage of  -ether linkages

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

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

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