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Handbook of Solvents - George Wypych - ChemTech - Ventech!

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21.1 Supercritical solvents 1447<br />

Reaction Catalyst<br />

Hydr<strong>of</strong>ormylation<br />

1-Octene Rhodium catalyst<br />

Propylene Cobalt carbonyl<br />

Olefin metathesis<br />

Ring opening metathesis polymerization Ruthenium catalyst<br />

Ring closing metathesis <strong>of</strong> dienes to cyclic olefins<br />

Diels-Alder reactions<br />

Ruthenium catalyst<br />

Synthesis <strong>of</strong> 2-pyrones Nickel catalyst<br />

Synthesis <strong>of</strong> cyclopentones Cobalt catalyst<br />

Cyclotrimerization <strong>of</strong> alkynes to substituted benzene derivatives<br />

Oxidation<br />

Cobalt catalyst<br />

Alkene epoxidation Molybdenum catalyst<br />

2,3-Dimethylbutene epoxidation Molybdenum catalyst<br />

Cyclooctene epoxidation Molybdenum catalyst<br />

Cyclohexene epoxidation Molybdenum catalyst<br />

21.1.4.1.2 Homogeneous reactions <strong>of</strong> supercritical water<br />

Homogeneous reactions carried out in supercritical fluids are reactions in supercritical water,<br />

organo-metallic reactions and Diels Alder reactions. Reactions in supercritical water are<br />

well studied 84 and will be described in the following section.<br />

Despite the higher temperature and pressures required in supercritical water applications,<br />

this solvent possesses unique properties that make it attractive as a reaction medium.<br />

Supercritical water has a lower dielectric constant as compared to liquid water, and the dielectric<br />

constant <strong>of</strong> supercritical water changes significantly with the density. Also, the effect<br />

<strong>of</strong> hydrogen bonding is less pronounced at supercritical conditions, one consequence <strong>of</strong><br />

which is the high solubility <strong>of</strong> organics in supercritical water. The reactions in the supercritical<br />

water medium are carried out in a single phase, which implies high reactant concentration<br />

and negligible inter-phase mass transfer resistance. Also, the ion dissociation<br />

constant <strong>of</strong> water is higher in the critical region and is lower as supercritical conditions are<br />

accessed. These properties also vary continuously in the supercritical region, so that they<br />

can be tuned during the reaction by changing the temperature and/or pressure.<br />

Examples <strong>of</strong> the homogeneous reactions are given in Table 21.1.11. In some <strong>of</strong> the reactions<br />

discussed in this category, acid or base catalysts were used to enhance the rates. 84<br />

Table 21.1.11. Homogeneous reactions <strong>of</strong> supercritical water<br />

Reaction Catalyst<br />

C-C Bond formation<br />

Phenol and p-cresol alkylation None

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