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Reactions under acidic conditions

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

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Reactions under acidic conditions

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  1. Reactions under acidic conditions Ian Suckling APPI 2012

  2. 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

  3. Hydrolysis mechanism

  4. Rates of acid hydrolysis • In practice, heterogeneous hydrolysis rate differences are even greater: • Cellulose 1 • Mannan 60 • Xylan 60-80 • Galactan 300

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

  6. 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

  7. 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

  8. Acidic cleavage of -ether linkages

  9. Acidic cleavage of -ether linkages

  10. 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

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

  12. 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

  13. Vertichem process – an example

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