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Reactions under acidic conditions Ian Suckling APPI 2012.

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Presentation on theme: "Reactions under acidic conditions Ian Suckling APPI 2012."— Presentation transcript:

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:  Cellulose1  Mannan60  Xylan60-80  Galactan300

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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