Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.1 Supercritical solvents 1445
Reasons for carrying out reactions in supercritical fluids:
There are several reasons for carrying out reactions in the supercritical phase. Naturally,
some of the reasons are coupled. Nevertheless, they, in general, relate to control, favorable
mass transfer and kinetic considerations.
• Enhanced reaction rate
The effect of pressure on the reaction rate and equilibrium constant at high pressures is
described in Section 21.1.2.4. As can be perceived from this section, supercritical fluids that
exhibit very high negative activation volumes for certain reactions will improve the rate and
equilibrium conversion of the reaction.
• Homogenization
Reactions that otherwise would be carried out in more than one phase (heterogeneous
reactions) can be transformed to homogeneous ones with the aid of supercritical fluids, so
that inter-phase transport limitations are eliminated. This is realized due to enhanced solubilities
of the reaction components in the supercritical fluids. Typical examples are reactions
in water (supercritical water can solubilize organic compounds), homogeneous
catalytic reactions, and reactions of organometallic compounds. Homogenizing one compound
more than the others in a system may also affect relative rates in complex reactions
and enhance the selectivity.
• Enhanced mass transfer
In many instances, reaction rates are limited by diffusion in the liquid phase. The rate
of these reactions can be increased if the reaction is carried out in the supercritical phase.
Typical examples are enzyme catalyzed reactions as well as some very fast reactions such as
certain free radical reactions. Selectivity considerations usually dominate in complex reactions.
If some steps of the complex reaction are controlled by diffusion, changing the
diffusivity changes the relative rates of the reaction steps and affects the selectivity.
• Ease of down-stream separation
Another reason for using supercritical fluids as the reaction medium is to fractionate
products, to purify the products or to remove unreacted reactants from the product stream.
Supercritical fluids can be used as either a solvent or anti-solvent in these instances.
• Increased catalyst activity
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.
• Tunable reaction rates through dielectric constant
Some properties of supercritical fluids can be monitored (manipulated) continuously
by adjusting the density of the fluid. Dielectric constant is such a property and the solvent’s
dielectric constant can influence the rate of the reaction.
21.1.4.1 Homogenous reactions in supercritical solvents - examples
Homogeneous reactions carried out in supercritical fluids can be either catalytic or non-catalytic.
The objective of carrying out both catalytic and non-catalytic reactions in supercritical
fluids is to increase the overall rate of the reaction by eliminating the inter-phase
transport effects.