Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.1 Supercritical solvents 1449
tance in the chemical process industries. As described in the next section, the advantages of
carrying out these reactions in a supercritical medium include enhanced inter-phase and
intra-particle mass and heat transfer and in-situ regeneration of catalyst. Catalytic supercritical
water oxidation will also be discussed. Other heterogeneous reactions that will be
described are fuels processing and treatment of biomass.
21.1.4.2.1 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 diffusivity 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 through the film surrounding
the particle into the bulk fluid. Rates of these steps can be accelerated utilizing a supercritical
medium for the reaction. Heat transfer effects are also important in a solid catalyzed reaction.
Higher thermal conductivity of supercritical fluids is an advantage as well. 88
For two-phase reactions (typically hydrogenation and oxidation reactions), the reaction
steps include the diffusion of the gas reactant to and through the gas-liquid interface
and then into the bulk liquid. This mass transfer limitation is also eliminated if the reaction
is carried out in a supercritical medium where the reaction takes place in a single phase. 88
Supercritical fluids bring other benefits to the solid catalyzed reaction rate besides
eliminating or minimizing mass and heat transfer resistance. Supercritical solvents have the
ability to regenerate the catalyst during the course of the reaction, which increases the catalyst
life and activity, since undesirable deposits on the catalyst, such as carbon deposits, are
soluble in the supercritical fluids. The rate of the intrinsic reaction is increased in supercritical
fluids and tuning the properties of the supercritical medium can control the selectivity. 88
Supercritical fluids may also bring opportunities in downstream separation of the reactants
and products. Examples of solid catalyzed reactions in supercritical fluids are given
in Table 21.1.12. 88
Table 21.1.12. Solid catalyzed reactions in supercritical solvents
Reaction Supercritical medium Catalyst
Hydrogenation
Fats and oils Propane, CO 2 Supported platinum, palladium catalysts
Acetophenone, cyclohexene CO 2 Palladium on polysiloxane
Fischer-Tropsch synthesis n-hexane, n-pentane, propane
Oxidation
Fe, Ru, Co/Al2O3, SiO2 Toluene CO 2 Co/Al 2O 3
Propene SC reactant CaI2, CuI, Cu2/MgO or Al2O3 Isobutane SC reactant SiO2,TiO3, Pd/carbon