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Handbook of Solvents - George Wypych - ChemTech - Ventech!

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1450 Aydin K. Sunol and Sermin G. Sunol<br />

Reaction Supercritical medium Catalyst<br />

Cracking<br />

Heptane CO 2 Zeolite<br />

Isomerization<br />

1-Hexene CO2, and co-solvents Pt/Al2O3 Xylene SC reactant Solid acid catalyst<br />

Benzene, ethylene, isopentane,<br />

isobutene, isobutane<br />

Mesitylene, propene,<br />

propan-2-ol<br />

Alkylation<br />

CO 2 or SC reactant Zeolite<br />

CO 2<br />

Disproportionation<br />

Toluene to p-xylene, benzene SC reactant Zeolite<br />

Ethylbenzene to benzene and<br />

diethylbenzene<br />

Butane, pentane Zeolite<br />

Catalytic Supercritical Water Oxidation is an important class <strong>of</strong> solid catalyzed reactions<br />

that utilize advantageous solution properties <strong>of</strong> supercritical water (dielectric constant,<br />

electrolytic conductance, dissociation constant, hydrogen bonding) as well as the superior<br />

transport properties <strong>of</strong> the supercritical medium (viscosity, heat capacity, diffusion coefficient,<br />

density). The most commonly encountered oxidation reactions carried out in supercritical<br />

water are oxidation <strong>of</strong> alcohols, acetic acid, ammonia, benzene, benzoic acid,<br />

butanol, chlorophenol, dichlorobenzene, phenol, 2-propanol, (catalyzed by metal oxide catalysts<br />

such as CuO/ZnO, TiO2, MnO2, KMnO4,V2O5,Cr2O3) and 2,4-dichlorophenol,<br />

MEK, and pyridine (catalyzed by supported noble metal catalysts such as supported platinum).<br />

89<br />

21.1.4.2.2 Heterogeneous non-catalytic reactions in supercritical solvents<br />

Use <strong>of</strong> the supercritical fluids as the reaction medium in synfuel processing is one <strong>of</strong> the earliest<br />

applications in the field. The advantage <strong>of</strong> the supercritical fluid as the reaction medium<br />

are again three-fold. During thermal degradation <strong>of</strong> fuels (oil-shale, coal), primary<br />

pyrolysis products usually undergo secondary reactions yielding to repolymerization (coking)<br />

or cracking into gas phase. Both reactions decrease the yield <strong>of</strong> the desired product<br />

(oil). To overcome this problem, a dense (supercritical) hydrogen donor (tetralin), or<br />

non-hydrogen donor (toluene), or an inorganic (water) medium is used. 63 Also, a supercritical<br />

medium provides ease <strong>of</strong> transport in the porous coal matrix. Finally, downstream processing<br />

(separation <strong>of</strong> the products) becomes an easier task when a supercritical medium is<br />

used. Kershaw 90 reviews the use <strong>of</strong> supercritical fluids in coal processing, while Sunol discusses<br />

the mechanism. 91<br />

The forest product applications in this category include biomass conversion 92 and<br />

delignification for pulping purposes. 93

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