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Handbook of Solvents - George Wypych - ChemTech - Ventech!

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21.1 Supercritical solvents 1451<br />

21.1.4.3 Biochemical reactions - examples<br />

Due to their tunable properties, supercritical solvents provide a useful medium for enzyme<br />

catalyzed reactions. 94 The mechanism <strong>of</strong> enzyme-catalyzed reactions is similar to the mechanism<br />

described for solid catalyzed reactions. External as well as internal transport effects<br />

may limit the reaction rate. Utilizing supercritical fluids enhances external transport rate<br />

due to an increase in the diffusivity and therefore mass transfer coefficient. Internal transport<br />

rate depends on the fluid medium as well as the morphology <strong>of</strong> the enzyme. Supercritical<br />

fluids can alter both.<br />

Water is known to be essential for the enzyme activity. Small amounts <strong>of</strong> water enhance<br />

enzyme activity, however excess water hinders the rate <strong>of</strong> some enzyme catalyzed reactions.<br />

The active site concentration on enzymes, hence the enzyme activity is found to be<br />

higher in the presence <strong>of</strong> hydrophobic supercritical fluids (ethane, ethylene) as compared to<br />

hydrophilic supercritical carbon dioxide.<br />

The effect <strong>of</strong> pressure on enzyme catalyzed reactions can be explained in terms <strong>of</strong> the<br />

transition theory. Supercritical fluids that exhibit very high negative activation volumes for<br />

certain reactions are expected to improve the rate <strong>of</strong> the reaction.<br />

Although, supercritical carbon dioxide has the advantage <strong>of</strong> being non-toxic and abundant,<br />

it is practically immiscible with water. Therefore, supercritical fluids used as the reaction<br />

medium in enzyme catalyzed reactions include fluor<strong>of</strong>orm, sulfur hexafluoride and<br />

ethane, while lipases are the enzymes utilized in such reactions. 95<br />

21.1.4.4 Polymerization reactions - examples<br />

Supercritical carbon dioxide is a promising green alternative to traditional solvents in polymer<br />

synthesis due to gas-like transport properties and liquid-like solubility. Supercritical<br />

carbon dioxide can be removed easily from the polymer solution by depressurization during<br />

drying <strong>of</strong> the polymer. Supercritical carbon dioxide provides easy separation <strong>of</strong> the polymer<br />

from the unreacted monomers and catalysts. Finally, supercritical carbon dioxide also exhibits<br />

Lewis acid-base interactions with electron donating functional groups <strong>of</strong> polymer<br />

chains. 96 Examples <strong>of</strong> homogeneous and heterogeneous polymerization reactions carried<br />

out in supercritical carbon dioxide are given in Table 21.1.13.<br />

Table 21.1.13. Polymerization reactions in supercritical fluids<br />

Polymerization mechanism Substrate<br />

Homogeneous, free radical/cationic polymerization Amorphous fluoropolymers<br />

Precipitation, free radical polymerization Vinyl polymer, semicrystalline fluoropolymers<br />

Dispersion, free radical polymerization Polyvinyl acetate and ethylene vinyl acetate copolymer<br />

Dispersion, cationic polymerization Isobutylene polymer<br />

Homogeneous/precipitation, cationic polymerization Vinyl ether polymer<br />

Transition metal catalyzed, ring opening methathesis<br />

polymerization<br />

Norbornene polymer, polycarbonate<br />

Butane, pentane, and propane are also used as the reaction medium in polymer synthesis.<br />

97 Furthermore, some polymerization reactions (such as polyethylene synthesis) are carried<br />

out under supercritical conditions <strong>of</strong> the monomer.

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