Heterogeneous catalytic reactions in supercritical solvents

Fats and oils Propane, CO2 Supported platinum, palladium catalysts 

Fischer-Tropsch synthesis n-hexane, n-pentane, propane Fe, Ru, C0/AI2O3, Si02 

Benzene, ethylene, isopentane, isobutene, isobutane CO2 or SC reactant Zeolite 

Ethylbenzene to benzene and dielhylbenzene Butane, pentane Zeolite 

Catalytic Supercritical Water Oxidation is an important class of solid catalyzed reactions that utilize advantageous solution properties of supercritical water (dielectric constant, electrolytic conductance, dissociation constant, hydrogen bonding) as well as the superior transport properties of the supercritical medium (viscosity, heat capacity, diffusion coefficient, density). The most commonly encountered oxidation reactions carried out in supercritical water are oxidation of alcohols, acetic acid, ammonia, benzene, benzoic acid, butanol, chlorophenol, dichlorobenzene, phenol, 2-propanol, (catalyzed by metal oxide catalysts such as CuO/ZnO, TiOj, Mn02, KMn04, V2O5, Cr203) and 2,4-dichlorophenol, MEK, and pyridine (catalyzed by supported noble metal catalysts such as supported platinum).  

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Supercritical Fluid Technology Reactions Table 3 Heterogeneous catalytic reactions in supercritical solvents 2921... 

Heterogeneous catalytic reactions in supercritical solvents Obviously, a solid catalyzed reaction takes place only on the active sites of the porous catalyst with the implication of some mass and heat transport steps prior to and after the reaction. The first step is the diffusion of the reactants through the film surrounding the catalyst particle to the external surface of the catalyst, followed by diffusion of the reactants into the catalyst pore to the active site in the pores. These steps are limited by the dif-fusivity and viscosity of the reactants. In the case of a supercritical fluid phase reaction, the diffusivity is higher than the liquid diffusivity, viscosity is less than the liquid viscosity and therefore, the rate of transfer to the active site will be higher. After the adsorption, reaction and desorption steps, the products have to diffuse out of the pore, and again... 

Some heterogeneous catalytic reactions are carried out in the supercritical phase, in order to increase catalyst activity and life through in-situ regeneration of surfaces with tuning of operation conditions. For example, supercritical fluids are capable of dissolving carbon that may otherwise be deposited on the catalyst in the absence of the supercritical solvent. 

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